I once read this H-Manga where the MC, his friend, and friend's girlfriend are talking. The friend has to leave. The MC ask if the two have had sex and she doesnt know what sex is even though the two live and shower together. MC teaches her sex and tries to kiss her but she rejects the kiss. Its not really NTR but its kinda unintentional cheating. (Warning: my description may not be 100% accurate)