Listen to Angel on this one, sounds like she's had to learn the hard way. I'd like to add that no relationship "requires" sex. If they "really love you" but then leave when they find out they're not getting any, they obviously never did love you. If someone really loves you, it's okay if he wants to have sex, that's natural...but he also needs to respect your decision not to. Once you've made it clear that you're not willing to take that step yet, then someone who really loves you needs to back off on that part of the relationship and be willing to wait until you're ready. Figure out who it is you really want in your life, and let them know what your expectations are within a relationship. If he's not okay with it, he's not right for you.
We'll add in some personal experience...I have a few friends who got married as virgins in their mid- to late-20s, and their husbands all agree that it made them respect them more that they didn't have sex ahead of time, and let them know that there really weren't any lovers they didn't know about in the past. I myself made the mistake of letting someone push me before I was ready. We did eventually get married and stayed together for five years, but by time I was your age I had a 2-year-old and was stuck in a miserable relationship. It kind of backfired on him too...he was really lousy in bed, and I learned that early on. Since he was the first I'd ever been with I didn't know just HOW bad, but it was really hard to keep my interest in that part of the relationship because I was never satisfied...and he didn't care, which now I know should have been a warning sign to me about his view on the rest of the relationship.