Yeah, but seeing that M&J scene was something that made that whole situation much harder. It may have happened before the MC was in the picture, but it's still a bit like watching a sex tape of your girlfriend with her ex; you acknowledge they've been in relationships in the past but you don't want to see it.
The fact is that showing this sort of thing could cause a "NTR" backlash from people who don't want to see their favourite LI get frisky with someone else, even if it's in a flashback, which is why I don't think that we'll see this here.
For some people it might, especially if they get a bit touchy-feely, kiss, or even get close to kissing.
I personally wouldn't call it NTR, but it's still not something I'd like to see, especially after the M&J flashback.