Thanks for the thoughtful comments.
strixish, since you asked -- the boundary I asked for was for neither of us to date for a while. The first person my partner tried to date as a secondary was a disaster, and it ended due to an important deception on his part. It ended about a month ago, and my partner raised the issue of trying again with someone new.
I am still too hurt and scared to do it again. His feeling on the matter is that I can do whatever I want, but asking him not to date indefinitely is too controlling.
I feel like it is a necessary boundary. I wish that I did not need it, but unfortunately at this point it is either that or leave the relationship.