Since by definition NTR requires stealing and sharing isn't stealing it wouldn't be NTR, unless it ends with (typically the girl though technically it could apply for the guy as well) being stolen at the end which I don't see happening in this game. Sadly even though to get on the sharing D with Martin route you have to make two choices, one of which involves watching an interracial porno as
RustyV mentioned, people still scream
bloody murder NTR when its their choices that got them there.
Either way if people ended up on that route but want to get off it you will probably have the option to get off it sometime before you would actually share D with Martin, and if we don't you can toggle it off by clicking on D's image in the top right and setting that route to off*.
*Unless the version posted here is completely different to what is posted on MrDots' SubscribeStar you can toggle routes off and on as you please (though toggling them all on might result in some weird stuff happening I haven't tested it).