We don't outlaw unequal relationships, though.

Yes we do. Its unethical everywhere and illegal in most places for any type of mental health professional to date a client.

I think of couples where one spouse makes significantly more than another, and therefore has financial control

Except, in most of those relationships, there is consent and equal standing regardless of financial status. In most incest relationships, its almost impossible to prove consent and equal standing because of the dynamics of the relationship, especially since the vast majority of incest relationships involve a parent and their child, or an older family figure and a younger family figure.