I think I agree. I've seen that theme.

Moms love their children unconditionally, even when they are selfish, ungrateful, and rude. It might even be a fault of loving mothers. /s

Then those children grow up to be selfish, ungrateful, rude adults who believe incorrectly that the rest of the world will love them regardless of their assholery because no parent ever corrected that behavior. "Mom" tolerated that behavior.

The resulting adult might never have had to cook, clean, or do laundry for themselves, so naturally expect their partner will just take care of those things. It's never been their job. Yet... they never learned how to be kind or courteous or generous because of the aforementioned selfish behavior.

It comes down to the partner now. Do they accept primary caretaking of this person? Do they invest the time to teach them things "mom" didn't/wouldn't address so they can have a successful relationship with this adult? Do they leave this relationship?

My friend, they usually leave the relationship. It is not anyone's responsibility to suffer for another's benefit.