Just thinking out loud here.....
If you believe Jesus died for your sins and that Gods word is law - and that the bible is God's word (albeit, filtered by the imperfections of man)......then if God's word says don't have sex until you're married, it should be a no-brainer and to say otherwise is basically picking a choosing what suits you now instead of really living based on your faith.
If, on the other hand you believe that the bible is more of a 'hand-book' to pick and choose the parts that suit you - as opposed to the devine words of the father, then I could see how this might just be open to a bit of interpretation.
Now for the fun stuff. Examine why something like pre-marital sex might be addressed in a 2000+ year old book (as opposed to it being 'upheld' later on):
If neither he, nor she (the two who will have sex) are banging the entire town - and remain faithful to one another, you're preventing STD's from ripping through the local population like wild-fire.
There may just be some common sense based on the times in the bible - whether you subscribe to Christianity or not.....and if I was to guess, it was probably easier to say 'God says so', than it was to explain science to the masses. True, control factors into the equasion heavily, but that's after the words were written - I would say the initial intent was more about preventing some rather unsightly sores in some rather inconvenient places :yikes:
However, I'll stop there because this opens up a whole world of insights that might not be shared by everyone
EDIT:
One last deep thought for those who consider the bible to be more of a hand-book than Gods actual words: If the bible is not the devine word of God, what solid proof is there that Jesus is, in fact, your savior and Gods true son? (because 'the bible tells me so' no longer cuts it.)
(Just a little light thought to ponder first thing in the morning
)