For the most part, people tend to agree with the major 'thou shalt nots' of Christian morality. Lying is generally accepted to be bad. Allowing people keep their own hard earned property, instead of stealing it from them, is obviously appropriate. Plunging a dagger into someone's back in a dark alley is universally considered unacceptable. The Christian stance on sexual integrity, however, is often seen as somehow outdated, irrelevant, perhaps even backward. Given two people, both mature and of sound mind, entrapped in fiery love with one another and wishing to consummate their passions, how dare anyone interfere? Perhaps most incensed by that suggestion are the young bachelor and the equally young lady who has her eye on him. Being faithful to your husband or wife is one thing, but who is this young lady to be faithful to?
Of course for the Christian part of the answer is God. God gave his rules, and the Christian intends to follow them out of deference to the rule maker. Even so it's only part of the answer because those rules are far from arbitrary but were given for our benefit. As with the rules against murder or theft, there are a range of benefits to reserving sexual activity until marriage.
The benefits to young women are particularly obvious: abstinence is the only completely effective way to avoid getting pregnant. Taking a romantic relationship to bed inevitably involves that risk, and that ushers in the terrible choice between killing her infant son in the womb or shouldering the burden of raising the child. The full depth of the abortion argument is better dealt with elsewhere, but it is always worth bearing in mind that the whole trap can be so easily avoided. The liberal young man willing to fight so hard for a woman's "right to choose" should really question whether he ought be giving her this poisoned chalice of a choice to begin with.
Similarly, reserving sex for a monogamous lifelong marriage is the best way to zero the risk of contracting any Sexually Transmitted Infections. Both the woman and the man share this benefit to themselves, but for every forgone sexual encounter they also avoid posing this risk to their groom or bride later in life. Each time a pair sleep together they play Russian Roulette with a revolver handed to them by the other, and they also loading additional bullets into the chambers of their guns. Some day the young lady will find a man she loves wholeheartedly, indeed a man she would like to marry. Would she rather hand him an unloaded revolver or one with 5 rounds in the cylinder?
This hints at the most significant reason to hold oneself to a standard of abstinence: faithfulness to husband or wife. The majority of people will marry someone easily within half a decade of their own age. They certainly aren't even thinking about such things by the time they're 5! This means that their future husband or wife isn't just some philosophical construct; they have already been born and quite possibly already been met. Many a lovestruck romantic will utter the words "You're the only one for me!" Can they line their actions up with their words? It's relatively well accepted that adultery causes problems. It often leads to feelings of betrayal and of low self worth. In many cases it's a major component of a very painful divorce, harming the husband, the wife, their children, and others in their community. If a bride found that her new husband, perhaps purely out of inexperience, is boring in bed she's liable to think back to her experience with previous more energetic boyfriends. At this her groom could justly be quite jealous, and so appear all the same problems as with a normal case of adultery. Just as faithfulness within a marriage is demanded, so the pair should bring one another faithfulness before the marriage as the most precious of wedding gifts.
Finally, it is worth examining the common excuse that a pair intend to marry anyway. Perhaps they feel it's necessary to check that they're "compatible". Precisely how this test works, or what "compatible" means, is never quite expanded on. What they'd do if the experience were less than they'd hoped is similarly skipped over. The truth is, a far better test of whether two people's love is strong enough to cope with marriage would be whether they're willing to surrender the chance for a little sexual pleasure just for the dream that is their bride to be. Any "compatibility" that depends solely upon their looks or performance in bed is doomed to grow faded, worn and boring. A foundation of honest, sacrificial love grants a marriage the potential to last a lifetime. That will certainly be enough time for them to gain whatever sexual experience they need, and importantly they'll gain it together.