All religions teach the sanctity of marriage.
Religious people who remain celibate before getting married enter marriage with the purpose of spending the rest of their lives with their partner and forming a family. That is the goal of the relationship and the expectations are the same on both parts.
Where relationships aren't entered into with the goal of marriage or long-term committment, it does mean that there is opportunity for fun (I mean fun here), but this fun is at the expense of the good of society as a whole in my opinion.
Chocs last post above points to the breakdown of marriages etc. She seems to cite it as a reason against marriage. I would view this as a result of the loss of moral balance in societies today which has weakened the institution of the family as a goal in itself and in which s.ex is a commodity to be freely enjoyed with few limits.
Marriage (and most relationships for that matter) involve an element of sacrifice and compromise for it to work. If one party gets everything their own way, the other party invariably is compromising in some way - that can work, but usually both need to compromise.
I'm not talking about the bedroom - but generally in all aspects of life.
If s.ex is confined to marriage, men have to commit to sticking with a relationship. S.ex is a great motivator.. it will ensure that compatibility in all other areas is as good as can be before a relationship is entered into.. which is, in my opinion, a better system than what many of my colleagues and friends practice - blind dates, drunken encounters in parties etc.
Friends of my age that are still single now rely on introductions by friends, or friends of friends to try and hook them up with long term partners. People of my community don't have to wait until we are in our thirties to realise that is the best way to find peace and a soul mate - to be introduced to someone who is compatible and who is seeking broadly the same things as you are. The difference is that we avoid the dating game and enter into long term relationships from outset, with all the highs and lows that go along with this.
Human nature being what it is, relationships sometimes fail despite all the screening up front. This is natural. That is why Islam recognises that divorce is sometimes the lesser of two evils (and incidentally why Islam was vilified as being too liberal by Christianity when it insisted divorce, womens rights etc were against the will of God).
People can choose to sleep around before marriage. My experience is that it 'cheapens' the act and works to the detriment of women - how they are seen and how they see themselves. I have daughters and I am acutely aware of this aspect of modern society and it concerns parents of all religions. The saddest scene I see is pre-pubescant girls dressing like their older sisters and mums in very revealing attire - the 'trashy' look.
We don't hide this choice from our daughters, but just point out the difference between how these women and girls choose to dress and how the women and girls in our communities choose to dress. Our in-joke in the household is that in summer all the poor people of the west come out in the streets - they are so poor they can't afford clothes that cover their bodies..
To sum up, in my view society is better off if s.ex is confined to marriage.
Wasalaam,
Shafique