Sex is one of the most important aspects of teenage life. Though sexuality is about far more than physical expressions of sex, there is no doubt that first intercourse is an important milestone in most people's lives. As Professor Peter Borriello of the UK Health Protection Agency remarks: 'It's increasingly the case that among young people a casual sh*g is part of the territory, it's part of life'. 
Professor Borriello rightly acknowledges the continuing change in sexual behaviour, but his further comment developing the acronym seems unquestioningly to accept that we should therefore react only to its consequences: 'Increasingly a sh*g now stands for syphilis, herpes, anal warts and gonorrhoea. If you are going to go swimming, dive into the pool, make sure you know how to swim, be safe. That really means wear a condom.'
Before considering these consequences, it is helpful to review the factors that pressurise young people into having sexual intercourse.
It is not difficult to see why many adolescents want to have sex in the light of the many pressures on them to do so:
In adolescence, the physical ability to make love arrives long before the psychological ability to be a mature, committed and loving partner. The mean age of menarché (onset of periods for a girl) is now 12-13 years in most developed countries, with minor variations,  and the earliest changes of puberty occur in both girls and boys at a mean age of 11.  There is strong evidence to show that earlier age of puberty is linked with earlier first age of intercourse. 
Never before have adolescents been exposed to such relentless media pressure to have sex as early as possible. The internet, cinema, television, pop music, novels and magazines all combine to project sex as a status symbol, and as the primary reason for living. One reviewer of teen magazines comments: 'Whether they are in a relationship or not, girls reading about boys' obsession with losing their virginity may continue to consider that sex is “the price of going out with a boy”.' 
The ability to separate sex from reproduction with a high degree of reliability and safety has inevitbly had a marked effect on sexual behaviour. There is considerable recent evidence that simply increasing the availability of contraception without accompanying education on the importance of saving sex  may lead to more sexually transmitted infections and unplanned pregnancies rather than fewer. 
There is some evidence that good school sex education, with an emphasis on delaying sexual debut, actually does delay the age of first intercourse.  However, the majority of sex education programmes in the UK have had no beneficial effects [9,10] and overall teenage sexual health in the UK is far worse today than ten years ago despite the millions of pounds invested in sex education.  There are though an increasing number of organisations providing high quality sex education. These recognise the importance of 'saved' sex (ideally saved for marriage, but at the very least for a committed loving relationship) as well as 'safer' sex (which usually equates to using a condom). These organisations include Love for Life,  Evaluate,  Oasis,  Lovewise,  LifeUK,  Challenge Team  and Damaris.  Their websites contain links to a wealth of resources which when combined give a truly comprehensive sex education package. Independence Publications  also produce some useful material for schools.
Sex and consequences
Professor Borriello recognised that, in the title of a sobering analysis of USA abortion policy,  there are 'Sex and Consequences' to consider.
Many of the intended consequences of sex are very positive. There can be a unique sense of pleasure and an enhanced sense of bonding and closeness between the partners. This bonding effect is one reason why, when two people enjoy sex together, God intends them to go on enjoying it with each other for the rest of their lives. This important aspect is considered in more depth later. However, the unintended consequences of sex can be catastrophic and the confidence of UK sex education in condoms as the solution needs to be examined very carefully. In spite of decades of condom promotion in the UK, accelerated by the advent of HIV-AIDS in the 1980s, sexual health is declining at an increasing rate.
Disease as a consequence
1. Sexually transmitted infections
The 2008 Health Protection Agency report on sexual health records in detail the continuing rise in sexually transmitted infections (STIs).  Overall there has been a 6% increase in the number of STIs newly diagnosed in the UK in 2007. There were 397,990 new STIs diagnosed in UK genitourinary medicine clinics in 2007 - up on the 375,843 cases in 2006.
Young people aged 16-24 continued to be the most affected group, accounting for around half of all newly-diagnosed STIs in the UK.
 New cases of genital herpes rose 20% while there was a 7% rise in genital warts and chlamydia. Young women aged 16-19 accounted for the highest number of cases of chlamydia and genital warts in 2007. Among men, the infections were most common among those aged 20-24.
2. Cancer of the cervix
Cancer of the cervix (the neck of the womb in women) is most commonly caused by strains of a virus called human papillomavirus (HPV) which is transmitted sexually. The advent of a vaccine against this has refocused attention on how inadequate condoms are in protecting against HPV and against herpes simplex virus (HSV), which causes genital herpes. These viruses are spread by direct skin to skin contact, rather than by body fluid transfer like HIV, the human immunodeficiency virus that causes AIDS.
Though an effective vaccine against the two types of HPV which cause 70% of cervical cancers is undoubtedly a great medical advance, it remains to be seen whether at a population level the introduction of the vaccine will lead to reduced death rates from this cancer. Concerns about such dangers as the complacency induced leading to falls in rates of cervical screening (testing for early pre-cancerous changes), and selective pressure on other HPV types to evolve into cancer causing strains are already being raised, and as yet we simply do not know how long-lasting the effects of the vaccine will be. 
3. Cancers and anal warts in men
The UK vaccination programme is for teenage girls and excludes males, despite the fact that HPV is the principal cause of anal and head and neck cancers in men who have sex with men. It is also the principal cause of anal warts  which, although not life-threatening, are difficult to treat and cause much psychological distress. (Anal warts also affect women similarly.)
Abortion as a consequence
In the UK, government-funded agencies now seemingly refer to increasing abortion rates as a positive measure of sexual health. Baroness Gould, chair of the Independent Advisory Group on sexual health, makes the ambivalent comment that a priority for sexual health is '...meeting women's needs in relation to abortion. We must continue to provide the services that women need, when they need them. We have seen an increase in NHS funded abortions as a result of the Strategy, and...we must support women so they have services that are safe and timely.' 
However, there is increasing evidence that abortion can and does harm women in the long term  and the Royal College of Psychiatrists has recently released a statement about this.  It is therefore worrying that the welcome 2% decrease in conception rates in under-18s, and the 1% decrease in under-16s, comparing 2006 with 2005, was offset by a 5% increase in the under-16 abortion rate (3.9 per 1,000 girls, up from 3.7) and a 2.7% increase in the under-18 rate (18.3 per 1,000, up from 17.8) over the same year. 
Psychological problems as a consequence
As well as being illegal in the UK (see box), sex at an early age is rarely emotionally satisfying, especially for girls. The founder of the US Sex Information and Education Council, Dr Mary Calderone, admitted that 'No one knows what effect sex precociously experienced will have...Sex experience before confidentiality, empathy and trust have been established can hinder and may destroy the possibility of a solid
permanent relationship.'  There is now considerable evidence that those who become sexually active before age 16 express more regret than those who wait until they are older.  Recent research has strongly suggested that early sex can cause depression in teenage girls. 
Other associated risks
Sexually active young teenagers are more likely to be involved in other risk-taking behaviour than are those who remain virgins. Sexual intercourse is often one 'element of a syndrome of problem behaviours that include drug and substance abuse, minor delinquency and school difficulty'.  One study found that sexually active girls aged 12-16 were over six times more likely to report having attempted suicide. 
The increasing use of drugs and alcohol is particularly worrying. Intercourse under their influence is much more likely to result in unplanned pregnancy or transmission of STIs. [33, 34]
What is the purpose of sex?
The Bible teaches that God's plan for human sexual expression is within the context of a publicly recognised, exclusive, lifelong, heterosexual and committed relationship - marriage.  The marriage relationship in turn is intended to reflect God's own relationship with his people, the church.  From this framework we can understand the purposes of sex.
Celebrating sexual pleasure
Whenever I ask a group of teenagers what the purpose of sex is, invariably the first answer I get is 'fun' or another word describing the pleasure of sex. Sex at its best can give unparalleled pleasure. Mohr perhaps only slightly overstates the case in suggesting that 'Sexual pleasure is...in intensity and kind, unique among human pleasures: it has no passable substitute from other realms of life. For ordinary persons...orgasmic sex is the only access they have to ecstasy.' 
The Bible too recognises this purpose of sex, with both its direct  and more indirect  expressions of celebration of sexual pleasure. However, this is not the only purpose of sex and indeed the primary purpose is so deeply embedded in the fabric of scripture that it is often overlooked completely.
Catholic writers have tended to emphasise the purpose of sex as procreation, reproducing the human race. With the huge contemporary emphasis on contraception, reproduction often comes well down the list of purposes of sex suggested by adolescents in the UK. This may have a bearing on our high abortion rate, as contraception so often breeds complacency about the safety of sex. Sex is far from 'safe' and God intended it that way. In the creation accounts in the Bible, procreation is clearly a key purpose of sex. 
On the other hand, Protestant writers have tended to emphasise the relational aspects of sex, on the basis that woman was created in response to God's declaration that it was not good for man to be alone.  In the New Testament, both Jesus and Paul quote Genesis 2:24 from the creation narratives as one of the key verses on sexual ethics: 'For this reason a man will leave his father and mother and be united to his wife, and they will become one flesh'.
This statement clearly links sexual expression with closeness in relationship, but within a context of a clear progression of activity. There is a public move to establish a new family unit. Then comes an exclusive heterosexual commitment to each other. Following this is the joy of becoming 'one flesh', a biblical concept which includes sexual intercourse within its broader compass.
In one of the other most significant biblical texts about the purpose and meaning of sex,  Paul insists that even when a client has sex with a prostitute, an invisible bond is formed. The two become one flesh.  For the Christian, there is no such thing as casual sex. A couple may have casual intentions, but there are always significant consequences including a change in the whole spiritual nature of their previous relationship. In Christian ethics, there is a unique relational aspect to sex.
Serving God together
However, in Genesis 1 and 2, both the procreational and the relational purposes of sex are set within the wider context of the task of serving God as his representatives on earth: 'The purpose of the man-woman match is not their mutual delight, wonderful though that is. It is that the woman should be just the helper the man needs, so that together they may serve...' 
This takes a little of the edge off romance perhaps, but I believe that is indeed the correct primary purpose of sex - that both in having children and in delighting in each other, couples should extend the kingdom of God on earth by serving him together. Once grasped and understood, this principle in fact enhances our enjoyment of being sexual beings for we are to use our sexuality in the service of God.
It turns the focus away from us (or me) and onto God and gives transcendent purpose for coming together (including doing so sexually), and for staying together, to prove in marriage the biblical truth that 'a cord of three strands is not quickly broken'.