Personally, I believe harem is basically the same thing as NTR. Specifically the subtag netorase (wife sharing). Wouldn't harem technically be the same thing but the genders are reversed?
No, NTR/harem are not two sides of the same coin, your example could easily pass for a harem with a female at the helm, nowadays termed reverse harem but would have a hard time passing for NTR since it's "sharing", if you swap out the term sharing for cheating it fails the harem test but does qualify for NTR status.
Harem implies that many people acknowledge that they're in a big relationship with one person at the helm(note: harems can be mixed gender and be helmed by members of either gender)
-if it's one person sleeping around without any official relationship it's not a harem, since no one acknowledges that a relationship exists it's just sex
-if one person tricks a lot of people into thinking they're in a monogamous relationship that's also not a harem that's just cheating
NTR is an umbrella term that means lots of very different stuff, it's not a comparable term
Netorare specifically what most people think of when people say NTR tends to imply someone cheating whilst leading on the person they're cheating on, these stories tend to either end up in break up after exploiting/leaching as much as possible from the person their cheating on or in the exploitation/leaching being indefinite. It's the manipulative elements and lack of player input that tend to turn people off.
-if a person really wanted to get with someone then finds out they were already in a relationship that's not NTR
-if a person really wanted to get with someone but that person wants someone else so turns them down that's also not NTR
-if a person shares their partner/allows them to sleep around that's not NTR
Tldr;
Harem is a type of relationship, NTR is a type of event they're just not the same thing at all.
