I'm going to platonically call my friend cute but then get called out by his anxious girlfriend and have it blow up
I'm wondering why you would want to call him cute at all while around his girlfriend?
He is my best friend and we are comfortable in our own skin.
But you didn't thought about how that wouldn't go well with his girlfriend? I don't think his girlfriend wants other to think that her boyfriend is gay and you shouldn't make her feel insecure about the relationship. She's obviously going to think you two might have something going on and you shouldn't joke around like that or you might cause your friend some kind of relationship headaches that he doesn't need.
Why am I not allowed to be a good friend?
You know that it would make you a bad friend to say these things around his girlfriend. Why make things awkward for both him and you?