Thanks to everyone for all the advice.
I'm abstaining because I don't want to sleep with a guy before I can be really sure he's committed to me. It has nothing to do with insecurities lol, I exercise and put a lot of effort into my appearance and am quite happy with the way I look, though I guess I get the sharp end of the stick when it comes to guys only wanting one thing from me as I'm quite girly-girl as well. Obviously no-one can ever be 100% sure a guy is committed, but at the same time if you're married to someone there is a difference to just being in a relationship with them.
I have a few close guy friends who've been with their girlfriends for 1 or 2 years, aren't exactly sure that those girls are right for them (to the point of asking me and other female friends for advice on whether to break up/take a break) yet are still continuing those relationships because they're afraid of being single (partly due to the lack of sex should they break up). I wouldn't say it's exactly using their girlfriends for sex (obviously they still care about them) but they aren't convinced those girls are "the one", yet I know some of their girlfriends and they believe they've found their Mr Right (we're all at uni) and think their relationships are going fine. I'd hate to be in a situation like that and sleep with a guy because I thought he was committed to me, only to find out he wasn't sure or thought there was someone better/more suitable for him out there.
Is it being too blunt to bring this up first thing should a guy ask me out? I actually often do have to have the whole thing in a club where you meet a guy and then tell them you don't do one night stands. But it's really weird if I ask someone out and then say "I won't sleep with you though, I'm not having sex till I get married."