@get Here's how I figure it, there's og versions of superman where he has super control over every cell in his body, even bone, and he can shift his bones and muscles to basically shapeshift. He's used it several times as a disguise. So I think he could use the same super control to fuck, and cum, at nonlethal speeds.
simpler answer: superman turns back into clark kent while naked
so he turns into softie simpboi who only does the slow rubs instead of beastly, womb-destroying thrusts.
my headanon says supes has a small peen anyway - dude's been wearing the same tights for decades and i've yet to see a fucking bulge