Perhaps the image was inspired the myth of Pasiphae. In a nutshell: Poseidon sent Minos a giant flawless white bull which was supposed to be sacrificed to Poseidon. Minos substituted a lesser sacrifice. Angry Poseidon cursed Pasiphae to fall in love with the white bull. Daedalus built Pasiphae a hollow wooden cow that she could hide inside to get the bull to mate with her, which it did, and thus was conceived the Minotaur.
Substitute horses for cows, bronze for wood, and a gray horse ridden by a naked guy (who could be Poseidon — who knows? — though the rider seems superfluous in this scenario in any case) for the white bull, and there you have it.
One must hope the horse isn’t put off by the two women’s calves and feet sticking out of its would-be mate’s thighs. I think Daedalus, the greatest sex toy engineer of the ancient world, would have done a little better.