You know how people ship the mane 6 with there gender bent counterpart , Well if the had kids like people make there foul or colt would be messed up because your inbreeding with your self!
Western society is kinda messed up when it comes to sexualization of siblings and things like that. Somebody somewhere always has to ship so-and-so with their brother or something gross. Inbreeding sucks. Incest sucks. This isn't Game of Thrones.
Eh...It's kinda tame in the grand scheme of things. It's shipping ponies with rocks and trees that concerns me.
You leave Tom out of this. In context yeah, but Game of Thrones has that whole historical reference thing going for it where it parodies real Medieval European events and ideas (Hadrian's Wall, the Norman Conquest, the Black dinner at Edinburgh, etc.) and also being weird for the sake of telling a story about weird people is probably okay. Fetishizing that weirdness seems worse.
It's messed up, but i just assume that they don't think through this and take it so seriously. Just for fun. They possibly don't even think about inbreeding and inbred babies.