I got quite involved in Christianity when I started university and there was a lot of pressure on me not to have sex. I can see why, the bible says not to until marriage (technically it says that having sex binds you eternally together and in God's eyes you then *are* married, or something along those lines). I can see that being a good idea when the Bible was written, as a woman couldn't support herself - they went from their father's house to their husbands - and without contraception she could get pregnant, not be able to feed her or it, etc. But these days? Some of my Christian friends are saving themselves for marriage (notably, these are the ones who haven't had a serious relationship yet...the ones that have have slipped up at least once and in a disastrous way as they hadn't prepared for it and bought contraception), they like the idea of no comparisons, of knowing each other have only done those things with each other, but what if their partner as been with someone else? Before you have sex it seems like a big deal, especially if they're spending years resisting, it would be upsetting to know they've got it elsewhere. Also I think the idea of one day you have to stand before God sat on a big gold throne and account for your wrongdoings is pretty scary and the fear factor helps people behave as they are told.
Personally... My virginity was gone before then anyway so I'd never have that special thing they're waiting for, a future virginal guy who waited for me would probably have issues with it and the idea of marrying someone having never had sex with them is mental. Plus my boyfriend is not a Christian so I'm already in the bad books anyway.