You showed massive insecurity when your girl told you about the party.

Correct answer instead of making an issue out of it might have been: "Cool, have an awesome time"

She complained to you about the grope, does not mean you knock him out. Make light of it, and give her a hug, tell her that only you're allowed to touch her sexy ass. Or something like that. Fighting him, only shows immaturity.

If you show rock solid confidence in the face of things like that, and display no insecurity she will love you for it. Going all needy and huffy for it, will make you lose control in the relationship and she'll go off you.

-Loke