So you think your gf is "slutty" because she fantasizes about threesomes and notices other hot guys? Wow... you do sound judgmental and sexist.
My boyfriend and I sometimes joke about how other people are hot and if they notice us we joke on how hot *we* are, but it's all as a joke, we know that we love each other and would never cheat on each other, it's just fun to poke fun at one another and I think it's awesome that we can comment on other hot guys/girls without anyone getting seriously offended. It seems that there is no trust between you (you certainly don't trust her), and most importantly there is no functioning communication between you two, otherwise you would understand each other perfectly.
Bottom line: you are a judgmental prick, she's immature, you two are incompatible due to lack of effective communication and different values.
I think you should break up and find someone closer to your idealized Madonna figure.