Sharing, yes, the MC let's others do things with the girls, but "NTR" (specifically netorare on this site), not at all.
"NTR" has to apply to the MC's perspective, not your own, so maybe some call it "NTR" because they personally don't want to share which is why they brand it as "NTR". And to clarify, sharing and netorare are two entirely different things, one is (willingly) sharing someone, the other is if a love interest (or anything into that direction) gets stolen from the you (in this case from the MC). And I specify again, the MC, it does not apply to yourself, so if some think this is netorare then they are wrong because the MC's perspective is not your own.
If the MC wants to avoid such situations he will, if not he simply does not care or even initiate them.
To go back to my first sentence, I clarified that on this site "NTR" means specifically netorare, but netori (MC stealing someone's interest) does apply here, but since it is excluded from the tag the tag does not apply here. Personally I think a clarification by changing the tag name from "NTR" into "netorare" is long overdue, as well as the addition of a "netori" tag.
These discussions already happened often enough, it rarely matters which game it is about, some simply confuse the meaning of "NTR" (the tag) or netorare.
Or maybe I haven't played in a while, but I am pretty sure it was always only sharing or the MC not caring and not specifically netorare.