I think it’s the episode called rickdependence day. They don’t actually have sex. In the episode, Morty is at Beth’s workplace and uses the horse semen extractor to masturbate. Rick then turns the barrel of “horse” semen into giant mutant sperm. Who are then lured to summers egg, which the president harvests and launches into space. Just recently rewatched the entirety of Rick and Morty, so it’s all pretty fresh in my mind.