Getting mad over some gross out humor seems a bit over the top. The whole show started as a “doc Brown tricks Marty into sucking his dick” joke, after all.
It's not really gross-out humor that made this episode bad - the team's made far more disgusting and grotesque creatures and storylines before; hell they almost had Morty get raped in a bathroom by a jelly bean, they're not really strangers to taboo stuff - it's more that they played the whole thing completely straight, and even forced in a feminism subplot that weirdly acted as the catalyst for the giant incest space baby in the first place. Like that was supposed to be an arc.
Rick being an underage horsefucker should be hilarious, but instead we get Rick actually getting feelings for her, realizes he got some easy underage horse poon at the end of the episode after she birthed a child that doesn't need to be taken care of, pulls out a Staples "That Was Easy" button, and that's how the episode ends.
That sounds like an ironic Reddit-tier shitpost, like it'd work if they played off how absurd it is, yet they played the whole episode as blasé as possible to the point where even Reddit (hell pretty much everyone) hates it. And when half of the jokes are recycled from other, better seasons, I can't really blame them.
EDIT: Same shit with the Planetina episode, albeit to a much lesser extent, since at least they just off-handedly addressed the creepiness in a “Yeah just ignore that bit, and you’ll be better off with this episode” kinda way.
EDIT 2: Also never mind about that Reddit comment, cuz apparently the whole fandom is split between it being cringe and the other side going “LOL R&M’s always been juvenile IDK just sit back and enjoy I liked it, See Justin and Dan even said they have incest fetishes so it’s cool.”
