The character or the person doesn't even have to be mind broken since getting lost in one's passion is a betrayal of one's fundamental character.
Ye. Standard moment of passion one night stands do happen and people do get ye olde brainrot. But when you have a character that aspires to be the greatest thief/mage/whatever or has a business or farm or family obligations that they have a track record of minding for the entire story up to the point where they get bedded and all of a sudden they only cling to the harem protag's arm and all of their development stops it's a solid no for me.
Where did their ambitions go? Why aren't they thinking about or even mentioning their family/friends/work left behind if even to spite their old life before they got so enamored with the protag? Especially dumb when the harem protag is either the doormat or pseudo alpha male archetype that also has absolutely nothing going for them.
Actually, the kind of protags that get isek'ied with nothing going for them but a stupid amount of power would realistically help their love interest(s) with their personal goals or get absorbed in their politics if they aren't actively trying to stake out on their own if their love interest(s) are not just booty calls, but we don't see much of that. The world just warps and a bunch of stuff doesn't exist in the narrative anymore because it gets smothered by egregious, gratuitous, personality erasing smut. If you're going to do that don't bother trying to present your story as anything other than just porn.
There
is a market for that and it's valid and whatever but don't promise me a good story with erotic elements then throw it all away for the latter. Just say it's porn.
If you care so little about your character's actual lives, why should I?