It’s a joke
Guess what, it’s not a joke.
Using someone’s sexuality as the punchline of your lame ‘joke’ isn’t funny and never will be. How would you like it if I said (not that I would), “Your a murderer? That’s so white.” Offensive, huh? And not very funny.
Why don’t you explain why it’s so funny to you and I’ll be better able to kick your ass in an insult competition.
Every time I use “That’s so white” in a conversation, the white people I say to to actually agree with me, so there’s that.