Hi Rachel,
I have not read all of the responses yet, but I'm sure from your post alone, you have received a lot of well answers, but I just wanted to answer anyway (smile).
The scripture from 1 Corinthians 7:36 does not justify pre-marital sex. What it says is, if a man has strong passion for a woman and he cannot control it (and some versions say 'and she is getting up in years) then marry her, it's not a sin. So, if a man is attracted to a woman but he feels his 'passions' are overcoming him (where he could sin if outside of God's will), then marry her. Marrying her will not be a sin. Paul said all of this AFTER he said that is good for man not to marry in this day and time b/c of all of the immorality occuring. But if a man has a sex with a woman outside of marriage, 2000 years ago or today, it is a sin.