Technically you're just following the story, so yeah, NTR isn't right, if she cucked him in front of him then it would be. Still, the first interaction with her is the one where you have the choice to lie and say that you slept together on the first date or tell the truth that it took time. I told the truth, because I wanted to see how she reacted, bam, "Yeah, that sounds like me." So, it is established in her first conversation that she is definitely not a slut. Any willing NTR should be impossible, unless it happens with her sister.