True according to the definition, you're right.
But I personnally don't mind if it's female. But male? NO WAY
No dick near my girls please
That's fine if that's what you like, but when it comes to definitions and the such, it doesn't matter really whether it's your personal likes/opinion. NTR goes for both men and women stealing your LI away. I like the occasional lesbian content every now and then (and in my opinion, it's too prevalent), but as the MC, I don't want the LI going off and leaving me for another guy or girl, or the LI being with another woman without the MC's consent.
It's why I both love and hate harem AVNs. Too many have unavoidable lesbian content between the harem LIs. A harem is a male power fantasy for a reason. It sounds piggish, but it's a harem.. the LIs are there for you, the MC, not to entertain each other sexually while they wait for the MC.
But, those are just my personal tastes. That's why if there's lesbian content, I always try to check to see if it's avoidable before I play it. Sometimes I'm in the mood for it, other times I'm not. Sometimes I want two of the LIs to be intimate with each others, other girls I want to be for MC only.
Anyways, TL;DR, that's why the tags have to go by the definition so personal preferences don't muddy the waters. It's why I just recently downloaded the game since I keep seeing conflicted reports about the NTR tag, even if it's girl-girl NTR. If there is any, even if it's a good game by the dev (which people keep saying it's really good, and I don't doubt it), I won't continue to play it, though I'll keep wishing the dev success on their game for those that do like it.