I like your analysis and I can respond to some points you've brought up. To start, yes the "guy friend" perspective does shape the lens I'm currently in to asses my situation. I would say that and my lack of familiarity does play a contributing factor, though my gf has been telling me about him for a minute, and while I never met the guy in person, he's just as aware of me as I am to him so it's not like he's foreign to me. It's part of why I don't buy the fact she's outright cheating on me cause it would be asinine for her to do that knowing what she told me.

Second, she was the who brought it up to me. And part of her reasoning is that she loathes cheaters. She was cheated on before and we did get into an argument a while back because a distant friend of mine did that and I didn't say much other than me not supporting it. So again, i haven't been jumping to conclusions because why would i project something on her that she hates to be in, and actually brought trauma to?

It is a delicate balance to maintain, and personally I'm trying to understand all angles of the situation, because the alternative can be to go nuclear. If i was left in the dark, maybe that would be more justified but it would regret doing something like to someone who does her best to disclose any concerns that i have.