I think either you misunderstood me or I misunderstood you, because I'm not sure what exactly you're disagreeing with or trying to say there.

Cheating is a bunch more than just betrayal of trust. I never said it wasn't. Emotional betrayal goes a lot further than eating at Taco Bell when you said McDonalds. Especially if you dropped the Chalupa, which is unforgiveable. The physical betrayal element just stings on a more primal level.
Was just saying for me
personally, I'd have a much bigger problem with an s/o cheating with a guy than a situation with a girl on a primal level. But if we hadn't already come to some sort of agreement that F/F was ok, it's still quite a bit more than Taco-bell/McDonalds... F/M would feel more like a physical betrayal (due to those primal/pride aspects) as well as emotional for me. But I'd still lose trust in her and could potentially see her under a different light in a variety of ways (including emotionally) in an F/F, especially if I was serious about the relationship. Unless you're just fuck-friends, relationships are far more than just physical. Especially if the F/F was more emotional between the both of them, rather than a sex-toy friend she might bring home to share with you. Discussed and agreed on beforehand, parameters and boundaries set = fair game.