Even better if it's his first time taking it up the ass, or even his first time at all. Needing to figure out what angle works best for him as s/o helps as well. Plus then he's seeing their face the whole time, they're a person and not some anonymous cock he's riding after giving into his desires. Helps make it feel all the more special, that it really is love and not just a guilty pleasure, perhaps even that it doesn't have to be sinful. The cross on his neck bouncing along every bounce and roll of his hips as he finds a pace that works for him, doing most if not all of the work but not minding it.