Also, if the human partner of the monster is submissive, it's not 'shameful', it's portrayed as appropriate and reasonable (the other is a monster!), and the human is less shamed for this kind of relationship. If the human is dominant it can be a power fantasy: the monster bows to *you only*.

Having a relationship with something completely inhuman, and so even the relationship can take some more unique forms, since it's not bound by human customs. That also takes off the burden of being a perfect partner, since the requirements are different now. It's freeing.