Being cheated on, but with a twist

So I wasn't dating this dude for long. So I'm not super upset over the fact we didn't work out and that he cheated on me. The main part that upsets me is that he cheated with a minor and he is still with her. Like after I confronted her about it and him, they harassed me about it. Very highschool like. I was sent pictures and videos of them together. I just wanted to be done with both of them.

So here's our ages to give some context. I am 20, he is 21 (almost 22), and the girly he cheats on me with is 17. Its pretty fucked up.

I really don't think I'd be as pissed as I am if she wasn't a fucking minor.