I think the difference is in control, ntr is without mc agreeing to it, sharing could be either all parties consent or without partners consent, either way the power dynamic is in favour of the mc. I'm down for sharing content in some cases but ntr has never been a turn on as I like to have control.
I had to learn to enjoy female protag, with male protag I kind of immerse myself into the role but with female protag I play from a more outside perspective, just following her experiences.