There are a couple other series that tie into the same universe as this one, and it's explained MULTIPLE. TIMES. that the custom for marriage is a duel of some sort and whoever wins cuts the other's hair.
Mpreg is probably only possible 'cause Suguri is a divine beast. He's more akin to a god than a demon.
Lastly, Seiju was literally in survival mode before meeting Suguri. He was also masking and hiding the fact that he's gay 'cause during the time he comes from homosexuality could be punished by death. He was also feeling guilty towards his parents 'cause he couldn't fulfill his filial obligations 'cause he's gay.
How he is after meeting and living with Suguri for a while is who he truly is. He doesn't have to live in shame or guilt 'cause of his sexuality, and other than the weird customs that world has, he's happy.
Well, sure, probably. I haven't read other works set in this universe, I'll have to trust you on that one, but it seems silly to have to read them before anything starts making sense in this one. I also feel this way about making a character do a complete 180° turn without almost any plot points to back it up and just calling it him finally letting himself be who he really is. Feel free to disagree, of course, that's just my opinion.
Actually I have to give you a point, there is now an information in summary that it is a spin-off, which I think wasn't there when I read it, because I genuinely thought it was a standalone.
I stumbled upon it because I really like the fantasy/forest spirit/demon trope, but is it only me that thinks that this was just… really, really, *really* forced? And frankly, a bit illogical?
*SLIGHT SPOILERS AHEAD*
Perhaps I shouldn't expect much out of this kind of thing, but what really bothered me was the way the warrior was portrayed in the most of the first chapter completely disagreed with the way he was portrayed after their "fight"? Also, a really stupid thing to mention, but his new haircut was 10x worse than the one he had before. I won't even mention how insanely annoying that demon was, but alas, that is just my opinion.
(And mention of mpreg was so unnecessary that I had to pause for a while.)