I've danced with a lesbian who was my girlfriend's friend before (my girlfriend was there, by the way). I'd always ever only seen her out as a lesbian but I got a really bi vibe from her in that moment. I think if we had been alone I could've taken it farther.
But yeah, the problem with a person giving the "well, they were gay so it doesn't count" argument is that it acts like bisexual people aren't a thing. Nope. They exist.
What is being described in the OP isn't cheating but it's disrespectful to their partner. It's a break up-worthy offense for sure.
I am I - you are you. These are irrefutable truths. Possibly.