Why do people assume this has anything to do with sexism at all?
This. It's not "sexist" so much as "homophobic", and probably "insecure". It's the same thing that makes people rage and foam at the mouth any time they see a character from "My Little Pony: Friendship Is Magic" anywhere on their monitor.
Is there visual proof that sexism is the reason here? At this point I think it's just being thrown around, because clearly, people don't have a problem with Pokemon like Togekiss and Gardevoir existing.