How about I tell my girlfriend I will leave her if she gets pregnant?
Seriously? Why would you leave your gf if she got pregnant?
I don't want kids and I still think of myself as a young man with a future ahead of him.
I know that you aren't ready to settle down, but threatening to walk out on your gf over a pregnancy is a wrong-headed approach. Talk to your girlfriend about your concerns rather than trying to instill fear in her.
I need to get my woman under control. She's a whore and I don't like all the cheating she does.
Threats are bad in any case, this is not the way any loving relationship works. If she is sleeping around then maybe you should just get another girlfriend too. But only if she is cheating, otherwise you have to commit to her no matter what.