Based purely on what you're saying here it hardly seems like treating her like a slut, it's just guys being overzealous and a lack of boundaries. I don't see how flirting with a girl = thinking she's a slut. Is it inappropriate given the situation? Yes. Sexist? Barely. Although one thing I will point out, and this is nothing to do with you personally, but I love how feminists always say how the gaming community is an "all boys club" and that women are treated like outcasts/mocked etc. when actually the reverse is true as proven, albeit anecdotally, by your comments about how they're far more welcoming to your girlfriend.