Is it fair to not like someone because they slept with someone you liked?

My bestfriend doesn’t like my other friend anymore because she slept with her good friend drunkenly on her birthday. Apparently they talked about it, and my bestfriend said that it didn’t matter to her. Subsequently, she has brought it up multiple times to me, saying that my friend shouldn’t sleep around.

Just out of curiosity, what are your thoughts on this? Why do women often blame the other woman instead of the person they like?