I witnessed cheating last night and I feel so uncomfortable.

Yesterday my roommates threw a “Galentine’s” get together, including one roommate I’ll call June and a friend I’ll call Mia. June has been in a toxic off and on relationship for years now with an abusive boyfriend she cant bring herself to leave. Mia has a boyfriend too, but he’s a really nice guy. June has been talking for months about how she has a crush on Mia, how gorgeous she is and how much she wants to kiss her. My roommates are usually very affectionate towards their friends, so I didn’t think it was too weird, but as the night went on June wouldn’t stop begging Mia to kiss her. Eventually they put on romantic music and started making out. At first the rest of us there were just watching, but then June yelled that we were ruining the moment and to stop watching. I ran out of the room and didn’t come back the rest of the night. My friends are usually good people and I’m shocked they’re just so casual about cheating? We’ve hung out with Mia’s boyfriend before and he would be devastated to know what happened. I feel like if you really want to make out with someone else that bad, just have the decency to break up with your partner instead of going behind their back. I kind of want to tell Mia’s boyfriend but I don’t want all my friends to hate me :(