First off, I'm not judging but 24 and 19 is a huge difference in terms of being monogamous. You're kind of approaching the age where you might be looking to settle down or at lease meet the person you want to settle down with. I doubt she is in the same place, honestly.
I also think you're setting yourself up for some big hurting by allowing this. People are people and even with the best intentions, cheating happens when good looking people are in emotional situations. I don't know your girl, but my guess is that in the right situations, emotions could possibly get the best of her especially being that she admitted that she is attracted to these guys.
The bottom line is, people in our world are so afraid of "making big deals" out of things, and end up regretting it. If it makes you feel uncomfortable, trust your gut. And tell your girl she needs to make a choice - she shouldn't be hanging out with other guys she's interested in when you're not around anyhow. Its just asking for it.
I wouldn't be a control freak, but I'd be honest and tell your girlfriend you think this is a bad situation and makes you uncomfortable. Maybe she will choose them over you, but wouldn't you rather learn that hard lesson now and not a few years from now? And don't you really want a woman who respects your feelings too.
I think she is saying some weird stuff to you, that I personally, wouldn't put up with.