There's nothing wrong with NTR, some people enjoy it. It's just a matter of taste. In my case, I don't like spending 8+ hours on a game chasing a girl for my guy, to then watch her fuck someone else and my guy cries pathetically on a corner.
It's worse when they had feelings for each other, but one of them gets stolen away. It's a pride thing, nobody likes having what's theirs stolen.
That being said, if a girl in question isn't happy in her relationship and my guy steals her away, that's fine. As you can guess, most of us are possessive by default.
Funnily, if the girls in "our" harem fool around with each other, that's usually acceptable too.
And, thanks for your good wishes. Hopefully you'll like HR.