As for the first part of your message, I imagined as much. Sorry I brought that up again, as a newcomer and I just wanted to voice my opinion quickly.
For me, it seems a really questionable turn of events that the only way to get an ending with one of the LI is having her fuck another guy in front of you. If that has been argued before, I completely agree with whoever held that opinion.
Regarding the second part and sarge87's quote, sorry I don't buy it at all. You don't need to have been able to start a relationship with someone in a VN to consider it NTR. To put forward a well-known example, when infamous Eric fucked Ann before Max's eyes the latter hadn't had a chance with his mom yet, but everyone considered and still considers it NTR. Perhaps the most notorious case of NTR in this community due to its repercussions.
But even if we assumed sarge's statement is indisputable, in Lancaster the MC has already had the opportunity to fuck Rose before that scene. In my opinion, that teory wouldn't apply. If that were the case, the only possible NTR in any game would necessarily involve the MC's girlfriend or wife.