I agree with a bunch of posters here, that sex does not equal love, and that it is something can wait until marriage. But, I don't think I could. I respect everyone that can, and I wish I could. But, having already had lots of sex, it'd be hard to go into a relationship knowing it wouldn't be there until years later. I have friends that have tried to wait until marriage, and they've all been cheated on. It's not right, but it happens. I wouldn't cheat, but I'd eventually find myself looking, and that itself is grounds to not be with someone.
If I did marry as a virgin, I'd feel pretty disappointed when it turns out that sex isn't all it's cracked up to be, at first. The first few times, I don't care who you are, are awkward as hell, and more or less, unfulfilling. So, it comes to thinking about comitting to someone for the rest of your life, that leaves you unsatisfied physically. You'll argue, bitch, complain, and end up not having any sex at all. And then, you end up looking at other people. And, you'd be in a position where it's best to just end it. As Gunnz not-so-eloquently put it, you wouldn't buy pants without trying them on.
My son's mother, was a girl that I started dating, purely in the hopes of just getting laid. I was 17, it was high school, and there wasn't much reason to be looking for the Future Mrs. Kidd. I wanted sex, and I found it. After a while, our relationship turned serious, and then we started talking about a family and getting married. Most relationships start out that way. You don't ask someone out, in hopes of marrying them. You ask them out, in hopes of connecting with them, and being happy. Marriage is close to being an outdated institution anyway (no offense Will).
Even with my current girlfriend...I wasn't planning on dating her when I met her. She found me through my friend's MySpace, and sent me a friend request. She was pretty cool, and we decided to hang out one day. Then we started dating, and the sex just happened naturally. We didn't plan it, we didn't talk about it. It just happened. We're very happy now, and have joked about a possible wedding in the distant future. If it happens, great. But, I don't think I could be in a relationship now, unfulfilled physically, looking at a marriage that may not come.