Romantic relationships where one partner kind of acts like a parent

I have noticed a few times between the couple that I know that one of them is actually kind of parental towards the other. I don't mean like doing chores for them or taking care of them in a material way, just that their emotional bond sometimes seems a lot like that of a parent and child.

I think this can be quite unhealthy if the couple is unaware of this dynamic, but I also see the appeal. There seems to be a little bit of overlap between the two kinds of relationship, and I can also understand that two people might want to deliberately pretend that they are parent and child sometimes for the sake of exploration. Somehow the idea of this seems to tap into something quite deep, so I think it can be fun and healthy if done properly.