You nailed it. He likes the romance aspect of Monster girl stuff more than the super-assertive reverse rape stuff. You said you don't like Succubus Academia but that game for me is the GOAT. Even games like Monster Girl Quest are too mild for me sometimes because you party with girls and they aren't constantly trying to rape you or coerce you into sex with them.
I really liked the Succubus Rhapsodia party system because you'd occasionally have to 'feed' the succubi in your group otherwise they'd eventually just attack and rape you. The really primal, animalistic, ravenous traits of the creatures resonates with me so if we're talking mythical creatures i like to think of them more as decadent vampires who see humans are little more than cattle to be sacrificed to them for even the most mild of pleasures.
If you remember the first succubus girl in Succubus Academia, she basically cultivates the first hero's love all the way throughout their childhood and beyond until she finally decides to pop him open like a fine, aged wine and drink his soul. This aspect of the genre really appeals to me because it really put the 'Monster' in monster girl. Envisioning all of the wild and crazy ways a monster girl might 'harvest' a man
really makes me excited, its not just about being killed though, being enslaved, milked and infested by a parasitic cum-hungry organism is also quite appealing, i am a fan of tentacle hentai for a similar reason.
Ofcourse the genre is big enough for the both of us, but it is rarely presented both ways in the same work. MGD is far too tame for me, SA and SP are just right.
As i said, we've previously discussed it at length on a number of different platforms. He likes Roses, i like Violets we both like Flowers.