He lies about the spell. It’s the fertility potion, of course, that’s causing this, but there is a spell with the same effect without the magical fertility aspect. Then, setting his eyes directly on Marvolo, meeting gazes without making his own slide away to lower the tension, Harry states firmly, "I want you to take the stamina potion and then fuck me into the mattress to within an inch of my sanity. §Please.§."