I can see SFM's point about accepting J's "termination power." Frankly, if JP has agreed to it, there isn't a lot she can do about it. I can also see how raising an objection to it now could backfire, making J feel like she's being ousted.
Whether or not SFM is fond of that possibility looming over her head, it's really there for any of us. While we may not have it as an official policy, most married people would tend to end the secondary relationship before the marriage. That's not to say it wouldn't come without a tremendous amount of resentment and anger... And I'm sure JP wouldn't go silently into that good night if J were to end the relationship. Whether or not J has that power "officially" I suspect JP would fight for his relationship. If J really put her foot down, I can't imaging JP not feeling resentment and anger over it.
So it's not like this power means that J could just on a whim throw SFM out the door, and that JP would just be like "Oh yeah, that's cool, I don't mind."
“As I am sure any cat owner will be able to tell you,
someone else putting you in a box is entirely different
from getting into a box yourself.” —bisexualbaker