We have now programmed the Monster girls having sex with each other. I'll explain briefly how this works. The monsters all have a Lust stat, which shows how horny they are. As idle behaviour a monster may choose to engage in sexual behavior if their Lust is high enough. If there is another monster girl within a certain range, they will attempt to have sex with that monster, however if the other monster's Lust is not high enough, they will reject the advance. Down the line this may result in a fight, but for now it just has them resume idle behaviour. If the other monster is down to fuck, the monster who engaged the sex will walk to the partner monster and they will both play the appropriate animation when they meet.
Alternatively if there is not another monster within range when sexual activity begins, the monster will simply start masturbating. Here is the video of the first segment of the first sequence. There will still be two more segments and a climax, as well as 2 more full animation sequences which are unlocked as loyalty is increased.