His Penis

Rarity's light masturbation as they have been talking has gotten her pussy plenty wet by now, and though he is tempted to get her off first with his tongue Spike's instincts to breed, even though his conscious mind thinks that to be an impossibility, override his chivalrous side and he lines up his long tapering tarsus with Rarity's cunt and starts pushing it in.

Rarity gasps and hisses as the conical member enters her, being used to male ponies who's shafts are much the same thickness throughout, but this different sensation is not bad, just new, and in a way better because... ~By all the gods, it's mobile!~ Rarity thinks in still further surprise as Spike's 'spike' starts bending and flexing inside her to touch and torment every pleasurable spot inside her entire vagina.

"Oh gods, yes! Spike, keep doing that: gods you're so good!" Rarity moans as Spike fucks her slowly with his magic dick, making her feel like no PONY ever has, and she knows in that moment that she had better learn to love this suddenly adult dragon leaning over her on her fainting couch and giving her an indescribable screw: because he has just ruined normal sex for her: after this it would be as bland and boring as unflavored oatmeal.