I guess I needed to be more clear in my first post about the situation.
What I believed Brian promised to refrain from, when he said, "I don't need to do that with her tonight," was PIV sex with his other girlfriend. I was fine with him doing whatever else he wanted to do to please her. I knew enough about her sexually and as a person to believe that she would be fine with this for one evening anyway. She confirmed that for me a couple of days later. The reason I wanted Brian to avoid PIV sex was that he has some pretty hardcore sexual dysfunction. That has, historically, been aggravated by him adding new partners. Twice running now, he has taken a new lover, and his dysfunction has worsened quite a bit with me, but he's been fine with the new person. This has been very painful for me. At the time that this incident happened, he and I had managed to have interourse in the month leading up to it perhaps once or twice, and probably tried over a dozen times. (Yes, we were getting lots of help for that, both medical and psychological.)
I knew that I couldn't just tell Brian, "No sex with her until you and I start to have it again," but that night I was feeling very low and needy and when he offered (I thought), to refrain from having intercourse with his other partner, it made me feel a whole lot better, and I let him know this.
I knew that Brian was having PIV sex with his girlfriend because he came into the room where I was with my boyfriend and actually got a condom out of a drawer, right in front of us, without saying a word about it. I could not believe my eyes. After he left the room, I said to my boyfriend, "Did he just get what I think he did?" Brian then proceeded into the next room, where his girlfriend was. He left the door to the room they were in wide open and my boyfriend and I could hear them going at it. If it had not been dark in there, we would have seen everything too.
Yes, he did obey the letter of what I asked, in that I didn't SEE it. But I sure heard it, which was just as bad. And otherwise, it feels that he was very insensitive to what I was going through, and I still think he was making a promise. I know I probably should have flat out asked him not to have intercourse with her, but we have been together a long time, and I really thought saying, "I am not comfortable with you doing this, but I don't feel I can ask you not to do it, so let me just ask that you don't let me see it" would be sufficient for him to offer to not to do it at all. And I thought that was what he was doing. He doesn't see it that way though.