an xh, his FF, Thanksgiving, MIL, and one psuedo-R

[Dom R]

Okay then. How do I get around the idea he has that I'm trying to force him to give up his stupid affair? I mean, yeah, that'd be nice. But what I mean is, how I present it so that it's about my boundaries?

Weeel.... for starters, point out that you're not holding a gun to his head. there's no "force" involved, so claiming that you are "forcing" him, is out of line.

he'll probably bring up the baby again, that you are "forcing him" by virtue of taking away him being able to be with the baby more. At which point, you have to be prepared to re-iterate, that you are not "taking away" anything from him. That would imply you are withholding something that he is entitled to. However, as two divorced people, he is only entitled to see the baby [whatever the agreement was].

If he would like to change the situation from "two divorced 'co-parenting people'" to something else, that is his choice to make, and you would be open to discussing that with him.
You might even go so far as to say that you would LIKE something better between you. It just has to be something that is mutually beneficial and acceptible to both of you.
Being close to someone who is dating someone else, is not acceptible to you any more.