1st off you have feelings for him or ur territorial. You prove different of trying to get them together when u outted him. 2nd he was never really sleep he was planning on treetrunking her if he hasn't already. So he heard u tell the girl that and I'm sure she texted him about it. If he has treetrunked her she kept the secret better than u. Sad to say but he probably wanted her more than u. & to add to it u didn't treetrunk him for New Year's Eve or that night so he lost. He is still going to pursue the other girl until he gets what he wants. He's using the tactic as he cut u off so he can get her. He's an unevolved young cap that just is selfish and is only looking out for his own sexual endeavors. He was going to probably wait until everyone went to sleep to make a move on her. And u r super friendly to have ppl in ur bed and not speak up. If anything I would have left her on the couch and text him and told him to come to the room. Or tell him to drop her off then head home with him. By u not sleeping with him makes it seem like ur chicken blocking. It's either u want the chicken or not...not when he's trying to give it to someone else. U SHOULD HAVE NEVER HAD ALL THOSE PPL IN UR BED!!!! Probably thought he could get a threesome. Move forward and let him be he's not done with his sexcapades. Once he hit her she's just another notch on his belt depending on if she's down with a detached fwb. How old r u guys? He could have been messing with her while she had a man. This is really messy. It doesn't matter if she had a man. Me and my bestfriend told everyone we were "bros" just that but we're secretly treetrunking. I think one of our other closer friends kinda knew but yea.
It seems like this is a pattern I've seen in myself and the Caps around me. It's more or less severe depending on the placement of Saturn in your chart. With Saturn opposing my ASC and in my 6th house you can guess how that goes for me. 🙃