@youdontknowwho I believe it's about the author of the series rather than the series itself. I'm guessing they did a lot of NTR stuff and then started officially publishing a manga so people just took to the natural assumption. I'm assuming all of this mind you because I don't know this guy's work as an H artist (nor do I intend to).
@Hitspark he does not work with NTR, people keep calling this NTR purely because they say the setting reminds them of this, even though there has been nothing of the sort in this manga every since the beginning.