This is a good point and one of the other things that can be an issue with monogamy (or at least the way it is envisioned in our culture). You are supposed to find your other half so that "two can become one" which really isn't healthy in any way, shape, or form.

I have no issue with those want to choose monogamy but that isn't what most people do. They fall into it by default because it's what is expected, and never have the sorts of discussions that would help them to really be in a relationship that isn't built around insecurities and owning the other person. Everyone, no matter their ultimate relationship style, should be working on themselves so that they can be healthy - for themselves and so that they can show up healthy in their relationship(s).