3 some gone wrong

So me and my girl-friend tried a 3 some w/ a guy I use to casually have sex w/ . Me and this guy had great sex, me and my girl had even better sex. When we tried the 3 some the guy could barely get hard. Only time he got hard was when I was giving him head. But I would say during this3 some the focus was more on me of course cause she is my girl and he had only been having sex with me but I just can’t understand why he was having a hard time getting hard. I literally got so mad and stormed out on him and her cause I just didn’t understand outt of all the ones me and him had sex the one time to score with a 3 some that he had been asking for since I met him ..couldn’t get his self up . Can anyone help me figure this out