I wonder if the real fear is that the feelings of the partner in therapy will change (and not for the better) towards the spouse (who is not in therapy)?

That's pretty powerful.

I think people have a sense of when they think the relationship is not as great / healthy as it could be. And, so when one partner goes to therapy, I wonder if the other feels that now it's just a question of time before the spouse is found to be inadequate and kinda left aside?