If it doesn’t bother him, then it isn’t cheating. She probably knew beforehand that it wouldn’t bother him. He may have said something to that idea in the past, I have certainly had guys say they wouldn’t mind if it was with another girl.
And I’d be way more upset if my guy had a secret emotional thing with some other girl than if he was having sex with one. But it would be reversed if people knew about it. Cheating is disrespectful, and if a boyfriend cheated and even one of my friends knew about it I would break up with the boyfriend immediately. Which is why I would never have an open relationship. I’m not sure what I’d think if a boyfriend cheated on me with a guy. I don’t think I’d feel threatened, which is the real reason I think people shouldn’t cheat. But I don’t think I’d believe they were straight ever again. I’d feel hurt that they didn’t tell me they liked guys.
Anything at or above cuddling is cheating to me. But I don’t consider porn or strip clubs or flirting to be cheating, but again, it would be disrespectful to do in front of me unless it is something we’ve talked about.