For the 5000th time, NTR is cheating on a love interest (and, at this site, with intent to cause jealousy). This isn't NTR in the slightest, it's never happening outside the boundaries of the F-D bond and what they allow for each other.
The MC has expressed love for other women throughout the game, btw - and he did it behind D's back (i.e., cheating). He's still in love with D as his grounding relationship and wants to marry her.
And, per prior discussion, D can love all different people in various ways, though her core relationship remains with the MC per every communication from the MrDots team. In the heat of the moment or from someone who you really click with, that can be "love" - for those who have lived through such experiences, the differences in degrees and ways of love tend to look varied.