And even after some recent nerfs, lust weapons remain OP as hell.
As for Hanzo's suggestion, I don't think that it would make much sense, both lore- and gameplay-wise. Items that PC loots from enemies are described as something those enemies carry with them, so why should it matter how you defeat them?
It will also mean that character builds focused around either HP or Lust combat will find it even harder to grind items that are linked to the other win condition.
I guess lust weapons are OP because to keep lust damage under control there's a damage cap to tease skills but it doesn't apply to lust weapons.
An explanation could be that some of their stuff broke during the fight. That being said, many drops don't make much sense already. Why are Kerokeras carrying a vial of their own venom, Zil Females carrying their own cum, etc.
The game is supposed to be easy, I remember something about attack being enough to beat the main quest, and that includes boss fights. This is only for random battles which should be easier, so beating them the way the character isn't build shouldn't be a problem. Keep in mind that this is to add options, encourage not to always do the same thing and cut some (boring) grinding, not to block stuff. I think I'll explain it better with an example:
Let's suppose you want a Vanae Spear. It has a 10% chance to drop. As things are right now if you want it you have no choice but to kill Vanaes again and again and pray for the best. In a scenario where the chance of obtaining it if you defeat her by lust is (for example) 40% instead of 10%, you can stil try to get it the "normal" way, or go for the lust win.
Speaking of builds, I don't like that once you've chosen a skill you're stuck with it forever unless you
cheat save edit, but that's a story for another time.