I really wish the game was more clear about what sort of erotic moves each monster could use before you choose one. I haven't gotten past the first day yet because I've been testing this, but based purely on the skills you can learn from the books in town, here's a general summary of each monster's theme. (By the way, each book is only for a certain type of monster, and their order in the shop is the same as the order here. The list is confusing, but each monster's books are in between the things that list the monsters' names.)
Slime: Your typical tentacles, engulfing, and aphrodisiac slime stuff, with a hypnotic confusion attack as well.
Bat: Hypnosis/confusion and beastiality.
Imp: Erotic magic and mind control such as charm and slavery spells, orgasm beams, and erotic dreams. Also has a spell that transforms the enemy into a playboy bunny (and seems to persist after battle).
Orc: Appears to be mostly violent rape attacks, with an aphrodisiac move and a hypnosis attack that seems to have only a 1% chance of working, so I don't know what it does. (Edit: If it works she's hypnotized into thinking she's a breeding sow. I think this might persist between battles?)
Homunculus: Seems to be themed after puppetry and body manipulation, with a few mundane(?) sex attacks.
Machine: Mostly BDSM (specifically restraint and sex torture) moves, plus one aphrodisiac gas move and a "common sense brainwashing" move that seems to persist between battles and cause flashbacks.
Flower: Mostly tentacle/bondage moves, plus an aphrodisiac move and one mind-blanking hypnosis move.
Phantom: Mostly possession, with one brainwashing/imprinting move that seems to leave a persistent "masochist" effect, although I haven't seen it do anything.