[Picus]

She wants to come. No, seriously, she feels left out. It's an insecurity thing. Unfortunately for you aside from assuring her you'll "be good" there isn't much you can do. Maybe offer to call her half way through the night to "check in".

Honestly I doubt there is any point trying to talk her into it. You could tell her she is being hypocritical because her offering an ultimatum definitely shows a lack of respect for your feelings; you want to be involved in your cousins good time. You could point out that you've been faithful and given her no reason to believe you would do anything untoward...but it won't matter, she will be pissed regardless. Go and have fun, and if she doesn't get over it she isn't worth it. I'm being serious, **** like this will haunt you later. It's a warning sign.

So ya, either way I'd definitely go to my cousin's bachelor party.