It does seem like he violated the agreements you made in the first place. If him having another partner makes it feel unfair to you if you aren't able to have one, then it seems like something that needs to be brought up. You are obviously not in a triad right now, you are in a V. Perhaps this is something you need to revisit with him. Remind him of what you agreed to in the first place and mention that you are not comfortable with him having a woman outside of your relationship that isn't involved with both of you if you aren't also able to have relationships uninvolved with him, be they male or female.
It IS a double standard and if that isn't something you are comfortable with, you shouldn't have to fall in line with it just because he is a little uncomfortable. Some people like having mono/poly combinations, but it doesn't seem like something you really want to do. He wants to have fun and make you do all the personal work to be comfortable with what he is doing, but he doesn't want to do any personal work himself and is restricting you and your potential relationships because of it. It isn't balanced and it isn't fair.