well i guess i'm in that situation now (except my bf doesn't seem too bothered). his issues are with the fact that because i've been messing round for the past couple months, i could well run off with someone else. but the fact that i've actually said yes to being this guy's girlfriend means that it aint gonna happen.
you can sleep with someone because you're drunk, on the rebound, anything. she probably regrets some of them.
speaking from experience, sex doesnt have to mean anything. and even if it did...god i dont even think about any of my ex's now. i cant remember most of them. and i really regret some. at the time, you dont think "well, maybe my future soulmate will care about this".
i had the same issues with my ex, he'd slept with 9 girls, i'd slept with 3 guys. i had such problems with it. in the end i realised, yes he'd been a slag, but the fact that i was actually his girlfriend, and he wasn't f*cking me over, and he was still around telling me he loved me three, six, nine months down the line meant he wanted me, and all the others were stupid drunken mistakes.