> Daddy pig is portrayed as an idiot.

I strongly disagree! It looks that way superficially, but he is highly resourceful and skilled. He's an engineer, a champion diver, and superb musician. In contrast, Mummy Pig is mostly a non-entity.

> All of the animals are married to animals of the same type: there are no "inter-racial" marriages.

Species ~ race is your own, silly, interpretation.

> perhaps due to exhaustion

Or because doing things badly is a core component of slapstick humour.