I’ve seen a lot. Animals do a lot of things. I’ve never been one to find the argument convincing that because a behavior is observed in animals that it is validated or a good behavior for humans. I’ve seen a dog humping a cat. I also observed what is more often the case which is smaller males being forcibly raped. Great apes same sex coupling is almost always tied to suppressing and forcing submission. I’ve never seen cats involved in gay sex, if it does happen it is probably not very common though is probably more likely in cases where the animals have no access to females.
From what the owner of the cats told me, it seems that adolescent cats sometimes get confused. They know they want to do the deed; they just don’t know who to do it with.