One way to raise a monsters Lust is to make them eat Neko Cum, I still need to do a few things to the command menu, since I'm aware sometimes the commands just don't go through for some reason.
I'm also thinking of increasing the Lust requirement for monsters to start masturbating, right now they won't do it if Lust is less than 50, but that doesn't give you much room to issue commands so I might change it to 75.