Unless she has sex with me, I'm going to tell my girlfriend I'm going to break up with her.
That's horrible, why on earth would you say that to your girlfriend?
It's just a joke. She will get the point after it's over.
That's not a very kind thing to joke about, people's feelings can get really sensitive and such a joke can cause more relationship problems than good.
Just trying out what happens. She'll understand it was a joke and won't be lame about it.
This isn't a joking matter and threatening to break up with your girlfriend if she doesn't have sex with you, joke or not, is cruel and manipulative. I beg you to not do this and if you still choose to please know I support your girlfriend if she chooses to end the relationship. This could make her feel very upset and uncomfortable.