|The definition for NTR (寝取られ） means you steals someones partner by sleeping with them. So a girl (or boy) steals someone else significant other by tempting them. This has a major sexual connotation with it.
With that being said there is no NTR in the series. The characters (at this point) all consider their partners so important that the issue is almost mute.
Just imagine what happens in most series based around a relationship. That is pretty much what Kimi Machi is about.
Spoiler (mouse over to view)
Yes, a boyfriend was stolen, but didn't have sex. Yes someone tried to sleep with someone else in a relationship, but they didn't.
"The point is that good deeds were done and we were nearby" - the idiotic Red Mage