I'm going to ask my newly ex-boyfriend if he is gay.
Who do you think he is gay?
I don't but I want to question him.
It's generally not a good idea to ask someone their sexual preferences out of the blue. If someone wants to tell you, they will bring it up. Asking them is invading their privacy and will make them uncomfortable.
He's my ex-boyfriend, so I can do whatever I want.
If you were together would that not mean he was straight? If you are calling him names to make him feel bad you need to move on and get a hobby or something.