If I remember correctly from the anime adaptation, Kelvin was portrayed to be a vegan - so no seggs like this happened. They just went to sleep together and Kelving just hugged her like an oniichan. (unless I missed the hint)
English isnt my main language, but if you interpret this "and it seems that the reason for that existence is reincarnated people who wish to have the power destroy the world" that way, that the demon king acts as a counter measure so the heroes wont cause too much trouble. Ofc this would raise the question why they are summonend in the first place. And obviously the godess is (partly?) responsible for that. What Im saying the world building doesnt make any sense in the first place.