Honestly most hentai universe insects and other weird beasts tend to look more aesthetically appealing than the overly bizarre ugly bastards. They're normally drawn so fucking hideous it's really eye-catching, I don't get how people can get off to shots of a girl getting railed with this nasty barely human-looking guy girning away in the background.
Yeah some of them are just stupidly, ridiculously hideous to the point they no longer even look human.
Socrates comes to mind, his main character straight up looks like a mummified corpse.
I always assumed it was because either a) they themselves look (or think they look) like the ugly bastard, and so they identify with him; or b) they got rejected by a hot girl once, so now they have a chip on their shoulder and want to see hot girls get their comeuppance.
I agree about the insects, though. It definitely depends on the species, but a lot of them have this almost alien look I find kind of cool. (That effect is 100% lost on me when it comes to deformed hybrid mutant babies, though.)
I like insects and I don't really mind the ugly characters either. I don't really have much of a problem with the ugliness, it's usually the "bastard" part of "ugly bastard" that turns me off. That's why I don't have a problem with insects, tentacles, monsters and stuff like that. The insects and the monsters in most stories are portrayed as simply acting on instinct. They're just creatures looking for a mate.
The "ugly bastard" character on the other hand is in most stories a gigantic psychopathic piece of shit who wants to completely destroy his victim's life, not just simply have sex. I can tolerate stories with ugly protagonists so long as the characters are not complete pieces of garbage. To me that just screams "edgy for the sake of being edgy".
So yeah, insect stories for me, preferably consensual ones. I like it when the girl is willingly degrading herself but they are rare. I take what I can get most times. Beggars can't be choosers when it comes to niche fetishes like this.