Would it be weird if I forbid my girlfriend to hangout with a guy she has hooked up with?
Do you hate the guy your girlfriend hooked up with?
No, I just don't want him to hurt her.
I can understand you not wanting her to hang out with him, but ultimately it is her choice. You can talk to her about it if you really feel a way about this.
I only said that I don't love him. I guess my words didn't make sense.
Talk to her and tell her your concerns. I am sure you can come to an agreement.