Indevi, have you considered a pacifist play through option ?
Similar concept to undertale in so far as not killing NPC's but using this games mechanics:
Enemy NPC's have an - initially hidden - aggression (for lack of a better description) bar that starts full, like the stamina bar - it can be depleted even if not seen but it will be guess work how close success is.
The bar is revealed once the characters sense stat has increased to level 5 and appears below the enemy NPC's arousal bar (maybe use the color red as that has been associated with aggression).
To decrease the bar the character uses a talk 'attack' - located below ero attack which is below the normal attack option.
The talk attack is the weakest offensive option starting out and remains below other attack types - starts out doing 1 point of 'damage' and the early enemy NPC's in the starting areas have between 8 to 30 aggression making it a challenging but not impossible option.
Managing to win by depleting the aggression bar assigns rewards as normal (attribute levels, Item drops) and could lead to lead to some alternate non sexual post battle options like petting the tiger boy in the park as an example.
I admit it may be a bit on the complex side but adding it later after sorting out all the core stuff you want to do first might be a solution.
And as an afterthought:
I don't think making enemy NPC's immune to a damage type is the greatest idea but I'm not against making some of them harder to defeat using different damage types.
As an example, As far as I know boss NPC's are immune to arousal damage which makes an erotic focus build worthless when fighting them ... so instead how about:
Normal enemy's (slimes, bandits, etc) can have a maximum resistance of 50% divided between attack types (the female bandit might be more resistant to arousal damage then physical damage as an example),
Boss enemy's can have a 75% resistance to one attack type and up to 50% resistance divided between remaining attack types (Sammy for example - 75% physical resist, 25% resist ero damage, 25% resist speech attacks),
Special enemy NPC's can have a 95% resistance to one attack type and 75% resistance divided between other damage types (the giant plant for example, if it could be defeated legitimately: 95% resistance to ero damage (instead of immunity), 75% resistance to physical damage, 0% resistance to speech damage (talking to plants IRL seems to influence them according to some gardeners).