Christians, why doesn't God support premarital sex?

I saw a question earlier dealing with premarital sex and a lot of answers seemed to claim that the bible does not support premarital sex because of STDs, unwanted pregnancies, etc. However, that is not always the case. If two people truly love and trust each other and want to experience their relationship sexually, why should they have to wait until they are married? I'm just curious to see different views on this. Some people don't have the time or money to have an official marriage, yet they know they want to be only committed to one person the rest of their lives. Why is it considered morally wrong for them to explore their sexual relationship together?