>>42561845
>>42561851
The reason it's taboo on earth is cause there's no other species with comparable intelligence to humans. That's not the case in Equestria, and I doubt you'd even get a second glance if a human and pony dated each other.
Spike is very obviously chasing Rarity's tail throughout the show, and other character reactions to that are generally "how cute" rather than "that boy ain't right." Granted he's a dumb little baby with no chance but I think the cultural reaction stands.