First, I just read the title and my immediate thought was: "That's fine, let him. His preferences don't dictate your own."
Then I read the rest of the post. Yup, same thought.
I'm a big stickler for keeping your promises and sticking to your agreements. If he was OK with this when there was something in it for him, then it's his responsibility to get OK with it again.
Being poly is too much work for him? Fine, let him be mono. That's his choice. But you can't put the genie back in the bottle.
Simply put, you tell him "No, I will not choose between you and Mark. My choice is both. If you can't live with that, you have your own choice to make."
I don't need labels to define me. They're sticky and I hate the glue they leave behind.