i think sex is supposed to be a form of deep trust that you show towards your partner. it shows to them that you love them a lot and would never leave them. i don't believe you have to be married to have sex; but i do think you need to be very responsible about it (wearing protection, birth control, etc.), and you should only go for it unless your partner accepts. if they don't, don't do it and try again another time.
still, it is also about creating new life forms, which does double the trust you put into your partner. usually this occurs after marriage (HOPEFULLY it occurs after marriage, or i'm going to beat somebody with a stick), and is trustworthy.
eh, it's a very iffy subject.