See, the problem is you're just thinking with your dick right now. The reason he feels the way he does about this girl is because part of what attracted him to her was her innocence. That's gone. You're like "yeah, just go out and bang some other pussy, you'll evolve." Into what? People are so messed up these days that they even get turned on by thinking about some other guy having sex with their wife. It's like they can't just care about their wife or girlfriend unless some other guy is violating her and degrading her. That's how pathetic people are.
To the OP: Yeah, at some point you'll have to come to grips with it because, unfortunately, most people have had sex with other people by the time you meet them. But that doesn't mean that it shouldn't bother you or that you should not care about it. And don't try to "match" their level of skankiness like "OK, she's had sex with some random guy, so I'll just have sex with some random girl and even it up and then I'm sure I'll have a fresh perspective on it." That's just that dude rationalizing his sexual promiscuity.