I'm going to give my girlfriend a nose ring ultimatum
Why do you want to be so petty and give her an ultimatum for a nose ring?
Because I think she should have a nose ring
Forcing someone to get one because you want it is pretty darn callous, ultimatums are for more serious moral issues. Let her do what she wants, it's her body her choice.
I hate her and I want to control her life and make her unhappy.
If you hate her as you claim you should break up and not cause harm. You can find plenty of single women with nose rings if it turns you on.