While that would be a reasonable reason to want to know, I don't think that's the OP's point. He never said that, and he gives to me the impression that he just doesn't like the idea that his gf may have a bit of a ****ing past.I think the OP's point is that if she's been ****ing herself self around, she's more likely to be less commital and less faithful.
Regarding the whole "double standard" of women being regarded as sl*ts while guys being regarded as st*ds if they have had a lot of sex, there's a reason for it.
For a guy to get sex, it's regarded as an achievement, and a woman is seen as "giving something up". This is similar to the view of "losing your virginity" held by some people on this very forum.
Pretty much any woman can get sex regardless of her personality, as long as she is not hideously ugly (and even that can often be rectified to a certain degree with make-up etc.).
Whereas a guy - even a reasonably good looking guy - usually has to have a certain amount of charm and wit, and usually has to do a bit of work in order to get sex.
OK this is not always the case, but more often than not, a woman has to be a lot less charming than a guy of equal degree of looks in order for them to get together and do the deed. So the woman is often seen as the one being gotten by the guy.
Now this could be to do with evolution (a woman can only have a child once every nine months so she should be selective with a mate, while a man can impregnate many women so he should spread his seed as much as possible to pass on his genes) or something but whatever it is, it's an old fashioned prejudice, and the sooner it's gotten rid of the better for men and women - men because it'll be easier to get sex, and women because there won't be the same stigma for them.
Women complain about the double-standards of the application of labels, (and by the way, I know a lot of women who apply these same labels), but I'd say that if you asked loads of women and men who they would agree to have sex with (honestly now), the women would be a lot more selective than the men.