I'm not going to let my ex-gf put me in drag makeup
What is your ex-gf's reason for wanting to put you in drag makeup?
She thinks I am gay and that it's fun
There is no reason to do something that you don't feel comfortable in doing. She shouldn't try to force or judge you, It rude and disrespectful. It's doesn't seem like she is respecting your boundaries.
She needs to understand that I'm not gay.
If you are not gay and you are not comfortable with this makeup than you really need to have the conversation with her. There is no reason for you to have to do anything you do not want to do.