Would you be okay if your gf/bf did this?

I've been dating my gf for about 10 months, we are both 19, in college and are sophmores. I've always been very trusting with her and never asked a lot of questions, I always made sure that she knew she could hangout with whoever she wanted whenever and that she always had time for her friends. Basically I didn't want to restrict her in anyway because i don't think thats something you should do in a relationship. 6 months into us going out she went to a dance party with a guy she knew from high school and ended up making out with him. She was friends with him and still wants to be but it's hard for me to be okay with them talking because it makes me feel like what happened doesnt matter to her. I've talked to her about this and told her how it makes me feel. She also goes over and hangs out at the frat he is in sometimes and I just don't know how im supposed to be okay with that or if I ever should be.