Well, unfortunately I can't get married to someone I love in most of the world because I'm gay. So waiting until marriage wasn't much of an option.
I have great respect for those who decide to wait and are able to do that. But I know for most of us, it's not something we're able to do, nor is it something we really wanted to do. Granted I did not lose my virginity until I was 25, for a variety of reasons. And I've had casual sex quite a few times since then when not in a relationship. Still, sex is something that I enjoy most when it's with someone I know well and care deeply about. I prefer sex in a relationship, but I also know that I have sexual needs that I need to meet even when I'm single.