Because to be the head of a noble family they have to fulfill the condition of having an heir, and demons can only have heirs (babies) with swans, since they can't impregnate, or get pregnant by, the humans from their own world. Moreover, there is a plus in that the baby born from the union between a demon and swan will always be a demon like their demon parent). Having a child with a swan is the only way for demons to get the heirs they need, since all the other humans from that world can only give birth to demons if they are a pregnant woman accidentally influenced by the dark magic that happens only during the night of the red moon, but that is very a rare and random event.
To sum up: demons can only be born through swans or randomly through the red moon event that happens only once a year (if the red moon gets to influence a pregnant woman)
And yes: both the ML and demoness were born due to the red moon, not through a swan, so their demonhood was a random event caused by the red moon, which ended up killing their (non swan) human mothers, as only swan mothers can survive giving birth to a demon baby.
Coorection: only "swan" mothers and *demon* mothers can survive giving birth to a demon baby (all other women die in childbirth, after their unborn babies get "demonized" by the red moon)
Wow thanks for explaining!! Now I get why the female demon is on the hunt of a human as well
Was it explained why Swans are hunted? Can someone remind me lol