I'm pretty sure there are 100 different ways for a woman to satiate a man without resorting to threesomes. She just wants to get NTR'ed, doesn't she?
She doesn't want him to just have a Threesome, she wants him to have a Harem or multiple Sex Friends.
The situation here is that, whatever she does, she can't satisfy MC's sex drive without breaking herself down.
I mean, yeah she maybe able to use 100 ways to satiate his sex drive, but can she physically do/use those 100 ways everytime they have sex? Be hand, blow, tit, anal, normal, etc, she can try it all, but in the end it would all be on her to do it all everytime for multiple rounds doing it alone, which is going to be a heavy burden no matter what.
I have heard of many break-up of relationships happening primarily because one partner couldn't handle the others sex drive.
Plus there have also been some cases were the guy in question had been legally charged for doing it too much, as for some reason in some countries men over-doing with their partner (primarily "wife" in this case) is considered as sexual exploitation and violence, because the man didn't restrict or control himself and went beyond the capability of his wife.
So the only other counter to this situation is MC restricting himself and only doing the amount which she can handle (aka., the idea MC already suggests).
And on topic of NTR, primary reason for it in majority of situation is dissatisfaction, especially sexual, be it for any reason.
But in situation like these were even the girl in question is also sexually involved with the Man and is the one who suggested the idea, the chance of NTR'ed is pretty low (unless the Author forcibly inserts it in this series).
Also one can say that she suggested Harem because it would reduce the chances of him leaving her. As rather than him ending up eventually cheating on her because of feeling unsatisfied as he was forced to impose self restriction to not burden her.
She is allowing him to fuck other women while keeping their relationship intact.
I mean, she likes having sex with him, and MC would get to have sex with multiple women. If he ends up leaving her (aka., her getting NTR'ed), then it's really MC's loss. I mean, why would you leave a partner who would allow you to have sex with other women (possibly without getting jealous or causing issues), over some attraction towards a single woman who might not allow that?