Long story short: my best friend of 7 years and I started dating 10 months ago. Everything is great except for the fact that his girl friend admitted to liking him a week after we were official and has been relentless ever since. She asked about 5 times if he wanted to go away with just her (he avoided answering until she dropped it, but didn't tell me for two months), she invites him overnight to parties with just her college friends, and this weekend he's going to stay over at her dorm after a party. He openly admitted to not seeing her behavior as malicious, defending her, and trying to be "sensitive to her feelings" without any regard to mine. He admitted that he'd rather me mad than her because "im less scary". I've had it at this point. What do I do?
He finally said something to her, but proceeds to go to her dorm on Saturday. I've been told to give him an ultimatum but I feel like that's wrong?
So he knows she likes him. So she going to try every dirty trick to persuade him. He is going to her dorm if there is going to be liquor, she might make sure he isn't aware of what he is doing, which could be dangerous. It's inappropriate, you should go with him to her dorm.
He's clearly hiding something or he enjoys the attention and wants to play her. Either way this is not acceptable behavior in a relationship. Pretty sure he wouldn't be a fan of this situation if the roles were reversed and you were acting like this with one of your guy friends. The fact that he'd rather cater to her feelings than yours, when you're his girlfriend, says a lot about who he prioritizes in this situation. Dump.
He can't even say no to a girl who asks him out when he's in a relationship, or tell her he's not interested. He keeps seeing her and tells you it's because he's scared of her. He's either a complete idiot or he's lying and he likes the attention/likes her. Either way I wouldn't want to be with someone who can't say no. I think you should be more determined about this, tell him you don't like his behaviour and that if he cares about you he better explain things clearly and tell you what his intentions are. If he's not willing to fix things he's not worth your time.