It's so rare though... So, say nothing unless the kid asks questions?
I mean, the rarity of it in kids shows alone is already going to convey a message that a minority of people are this way, most aren't, but some like them do exist and that's fine and nothing for the rest of us to be a jerk about.
Kids shows aren't going to magically make kids gay just by showing a same sex couple, that isn't how life works.
Would you rather children grow up to react with cruelty, toward people who have the right to make their own life decisions, including gay relationships, because they were given that message instead that it's okay to be abusive toward those who don't conform their own life to meet your comfort level?