Normally I wouldn't say anything.

But it depends on the guy, of course. I've seen gay guys that date transmen and go around saying that other gay guys should also date transmen. They use themselves as examples, saying "see, I date a transman and I'm gay". In these cases it makes me want to say something because these people are actually damaging other gay men.

In these situations I'd say to flip it as a positive thing. Like "it's so cool to see you embracing your bisexuality".