I'm intentionally gonna do something that will piss my girlfriend off.
What are you going to do?
Well, her biggest fear is her boyfriend cheating on her. So...
Are you implying you're actually going to cheat on her? Why would you do that to her? That's extremely cruel and will cause her so much pain.
So what. I'm just gonna do it instead of thinking about it.
If you're going to do something as selfish as cheat on your partner just for fun, it may be time to instead end the relationship. If you just end the relationship, you two can move on and find new people. This will avoid the pain you will cause her by cheating on her.