Well like I said, I realize it isn't a popular sub NTR fetish to have. Also, I realize that most (all) Yuri scenes in anime is just fanservice for the male viewer. The Bakemonogatari one felt different because, being Shaft, they emphasize every little detail and it felt more like how a sexual encounter between two women might occur compared to the usual, "Your boobs are so big let me squeeze them."
There is also an amount of detachment from my part in these scenes where if you don't see these as fanservice and if it actually happened in the real world your would have a different opinion of the events. If my girlfriend took a shower with another girl without me, or if she was stuck in the closet with my niece and they fondeled each other and kissed, I would be like, "What's going on?"
I typically relate things to my real life too when it comes to NTR so that is probably part of what I'm doing. My wife's best friend I am halfway convinced is a in the closet lesbian. Sometimes they go to conferences together and share a hotel room and bed. My wife doesn't think her friend is gay and I trust her implicitly, and she is 100% straight, but my imagination can of course get the best of me, and if something ever happened I would be in an NTR situation myself.
To go with that, in real life, if someone in a straight relationship, has a gay affair, well that pretty much confirms they are gay and not straight, and your chances of winning them back is even worse then if a man stole them from you, IMO.
And to cap this all off, of course regular NTR is my brand of choice. Most of this comes from desperate times, desperate measures in regards to the utter lack of ANY NTR material.