Uh ... [ Being a japanese teenager from the 90s, fluidity of gender is a foreign concept to him. If it was just a joke then that's ... more comforting? Still, while Josuke does like to crack jokes he's never joked about such serious and concerning topics. Of course he would be worried hearing something like that. ]

Of course I'm going to be worried, hearing someone say that. Not that I know a lot about this stuff, but I'm the kinda guy who'll wanna be there for a friend when they're feeling sad, y'know? [ Yeah, they are immediately friends. Deal with that, asshole. ]

But okay, if it's really just a joke ... [ He'll let it go. Even if he has some suspicions it's one of those joking-but-not situations. ]