Absolutely, so long as the sex is completely and unconditionally consensual between both people and that it is safe sex. To me, sex shouldn't be saved for marriage, it should be saved for when you're in love. Marriage to me means that two people love each other so much that they have made the decision to officially "seal" their relationship, at the highest point of love, so that they may be together "till death do they part". Adultery is different though, that's never okay!
But it is okay for you to believe in staying abstinent until you're married. It's a personal choice, and one that is highly praised. Don't EVER feel pressured by other people to do something you're not comfortable with!