By the power and law of magic and fantasy, he will not die; Just as he won't die from the obviously too large D of Caius that would have destroyed his body by now, even with the "light" extent he's screwed him already.
I suppose that Kouichi essentially becomes Elasta-girl every time they have sex.
╮( ̄▽ ̄")╭
Again, that's where that elasta-girl body comes in handy. Stretchy, yet snaps back to proper form when you need it to....Like spandex(๑•ㅂ•)و✧
It's the world of manga, they'll find a way one way or another to force it to work. They already have magical summoning traditions that spans across universes and dimensions, I'm sure there's other magic rampant in this world.
you guys realise if he got prego and gave birth he'd....die normal birth is the most painful thing in the world some people die from it so if he gets prego there is a 99% he'd die ,right...... no only me ok