<human>: Condense the text into a few key points

So I [m21] have just found out that my ex GF [20] is sleeping with my "friend".  I told my best friend about this, who is currently friends with the guy she is sleeping with, and he said that it "wasn't his problem" and that he "didn't want to get involved", even though he thought it was a dick move to make.

I expected my best friend to have my back, and be (somewhat) as angry as I am at the guy she is sleeping with, yet he "doesn't want to lose a friend over this" and is distancing himself from it by saying "it doesn't directly involved me so I don't want to get in the middle of it". 

I am pissed at him because he is basically condoning our "friend's" actions by not letting his scumbag ways affect his relationship with our "friend". He said he would be fine with going out for a drink with him next week, for example. I dispute his position on moral grounds, saying that he should be pissed at our "friend" for breaking such a serious friendship rule: don't sleep with your friend's ex, and he calls me childish for wanting him to do something about said "friend's" actions, ie, not be friends with him anymore. After all, how can you possibly consider someone who does something like that a true friend? I'm interested to know what the rest of you guys think. If you need more information before passing judgement just ask.
<bot>: I am angry at my best friend for not being pissed at our "friend" for sleeping with my ex, 1 month after we break up. Am I overreacting?