Idk if I conveyed it correctly but at least as far as I know, Yuuri's friends are very much messing with him. Also brocon/siscon jokes are pretty popular in anime and manga, so I assume the author just thought it was funny.
As far as Yuuri's sexuality, he seems to not care, but it's also possible that he hasn't met someone who "gets" him regardless of gender. That's what his dialogue implies, he wishes he was a little more open.