>>74532263It's just been the fans from the start, specifically the underage Tumblrite/DeviantArt demographic
SPOILER:
The absolute worst that Undertale itself is guilty of is having a couple gay romances, but a) it's mostly played for humor and not a huge deal, and b) it's been a while since I looked deep into the lore, but iirc the "monster" species doesn't reproduce sexually so it isn't really meaningful to be "gay" 