It's not just lacking art, it has no effects in battle or after-battle dialogue. The grapple system itself is also annoying and more likely to bleed out then give you time to fuck them, the easiest way I've found is knocking down the Adventurer while being erect, being in power-stance, then fucking him during his doggy-style stance before he gets back up.
The ones that you can fuck are humanoids like Harpy and Spider-Girl but without exotic lower bodies such as the Centaur
Actually, all you need is taunt 3 to 4 times to get yourself erect, then make them loose their footing and when you are in an offensive stance with them on the ground, you can either facefuck them or grapple them. facefuck is straight forward enough, but when you grapple you have to grapple a few times to get the prompt to penetrate, with it you can go into misionary or flip them and penetrate. You cant do that to all enemies and some enemies will give you a hard time once in a while, but thats pretty much it, i havent tried the doggy styled one, ill try it next.
The game does lack artwork, battle effects and after battle dialogue, but other than that you can rape enemies, even though most of them will actually let you rape them once you get into position