Maybe pay a visit to panda or your preferred doujin site and read Koyama's two Mamange doujins, and you might understand what I mean. It ain't the sex I'm bothered by, or I wouldn't have read them in the first place. They have some distinct NTR vibes, which isn't really how I wanted to imagine Mamange Youmu finally pairing up with Reisen
I've read those doujins. It's pretty obvious that they're not canon to the story given how wildly different Youmu's personality is.
Not canon, no, but it's still hard not to think about Youmu being used as a stereotypical NTR male in a setting from one of the most wholesome 2hu comics there is. It just kinda sours the idea of them actually being a couple now