I'm going to break up with my pregnant girlfriend
Are you leaving her because she's pregnant?
No, I just want to sleep with someone who isn't pregnant.
You should consider how much she needs you right now. That's pretty messed up to abandon someone who's relying on you. You are part of the reason why she is pregnant.
Fuck that, it's not like she needs to be with me forever.
If you are having a child together, that is a big commitment. You may not be together forever, but you do have a responsibility to help raise the child before and after it is born.