I...I will just preface by saying this is not my vibe and kind of had the feeling this wasn't my vibe.
This kind of reminds me of "I'm Dating a Dark Summoner" in being that semi-common sort of "hentai comedy" that only is funny if you are arm deep in that sort of stuff. The distinction I think is that manga references things that the audience might be aware of but find unpleasant, but it doesn't go all the way. The priest and the summoner are
never shown having sex with anyone besides each other even if it's referenced, and I think that's the key to having more of a broad appeal. This references a hentai trope
and goes all the way, and that is going to be a bit too much for anyone sensitive to this type of stuff. Personally? This doesn't make me blink on an tolerance level but damn is this uncomfortable.
Yes, let's keep moving the goalposts and convince ourselves this is NTR.
Kind of an interesting topic; I've noticed a lot of people here are quick to call everything NTR and...honestly I'm wondering if it's sort of a case of projection, or "Seven degrees of NTR". This scenario isn't cuckolding, or ntr, or whatever, but it's a certain number of degrees away from the ntr or "ntr" scenario where some young child "steals" their friend's mom/aunt/etc. due to being bizarrely good at sex (honestly I feel a bit blegh saying that and I can get why someone wouldn't like that). As a generality, it also could be that (and I'm assuming most of the people reacting like this are guys) a lot of guys are very sensitive to the sheer concept of a woman describing how they had sex with someone other than the main character (especially if they are projecting onto the main character), just as guys are often sensitive to stuff like this irl as well.
So I think this is a case of misinterpreting the story: I don't think you're supposed to project onto this Demon Army mage guy that hard, or the demon army in general; if you do, I'm pretty sure the joke is more about how uncomfortable it is to hear your parents talk about their sex lives than it is about feeling personally attacked. That said, the angle where it's a young boy is pretty damn uncomfortable and I kind of hate this. Replace them with a similarly aged man to the mom and I would maybe like this a bit more depending on how it was implemented.
TL

R Lay off on the hentai folks.