I'm thinking about cheating on my girlfriend with a pregnant coworker, but only temporarily, because she is also engaged.
Why are you cheating on your girlfriend?
I feel unsatisfied in our relationship.
Even if you're not completely satisfied with your relationship with your girlfriend it's wrong to cheat on her. Think of how devastated she will be if she finds out.
I don't care. She doesn't satisfy me and she has to understand.
If you really feel no more love for your girlfriend, and you expect that she would understand, then you should be honest with her.  Let her know that you think the relationship is over.  If she thinks you are still with her, it is wrong to cheat, even if it doesn't mean anything.