I really, really don't want my girlfriend to wear a revealing bikini
Why don't you want her to do that?
I'm a man and I want to be the boss
She is an adult. You don't really have the right to tell her how she can dress.
You don't understand that I want to control her and I want to be in control of what she does.
I think you have to step back a bit and put yourself into her shoes. How would you feel if someone forced you to do things only they wanted to do? Your girlfriend is her own person and can do what she wants, so it would be best to let her express herself to enjoy life.