I mean, he's an extremely tall very jacked guy who works out all the time and is hung like a dragon. She's like, 4ft tall with no known athletic ability. It not a mystery who will probably have more endurance.
Not really... Bedroom dynamics isn't about endurance, and he has shown time and time again his struggles about intimacy and the way he feels safe around Uzaki. The problem here is that his father's BS is trying to make him something he's not.
To put it simply, he might be a power bottom... But still, that doesn't make him a top.
I don't know if this is intentional writing or not, but fact is, his father has put up a facade of being a womanizer despite being married with a woman that completely kicks his ass in more ways than just in the dojo... The guy is full of shit, and I'm pretty sure that if it wasn't for his facade, he'd behave exactly like his son. Perhaps that explains why his wife treats him the way she does.
Still, I don't expect the manga to follow what I just said, because it's not formulaic enough for a shounen manga.