I read on this site that the pokemon that comes out of the egg is the female, with moves of the male, I did the breeding with a Marill and a Clefairy with Sing, and the hatched Marill only knew Defence Curl and Tackle... why didn't it learn sing?! And will it learn Sing if the male knew it?
Its because Sing isn't one of Marill's Egg Moves, Which are moves that a Female pokemon's egg can inherit from it's father.
Look Here for a list of moves that Marill CAN learn, and It's egg moves are included under a little section, Titled, Egg Moves.