If it had been a joke the father could have said something like “Time flies when you’re a fairy”.

I know that wasn’t your point, but your first line made me realize that. It’s still fucked up, but if you have a good relationship and know what the person is ok with that could absolutely be mistaken for a bad joke. Instead he just said something idiotic.

I don’t participate (feels kind of gross), but I’ve seen people joke with each other using slurs about the other. But I’m not the language police I’m not going to tell people what they can and can’t say. But this wasn’t even something that could accidentally be mistaken as a joke. This was just someone looking for a way to respond with a slur.