I understand how the enemies never respawn after defeating them to avoid xp farming, but, with no respawn, death isn't something one really needs to avoid. This takes away too much from the challenge, in my opinion and, even more important, if one didn't get a sex scene with an enemy, one just won't get it in the future.
I suggest to make enemies respawn but drop no exp after the first defeat (unless one didn't get some of it the first time, in which case, it should drop that one). Maybe some indicator over their head or some border around them could make it clear when they no longer have exp to drop