The Nunh of the U tribe is the leader of the Tribe, but even he admits that that is an unusual situation, rather then the normal one. Meanwhile, the M Tribe was large enough that at one point they had two Nunhs.

From what we learn from the M Tribe, saying that the Nunh is the "breeding male" is selling the role short. While it does involve that, it also involves everything else a father does in bringing up kids. That is, the Nunh is supposed to be the father of all the children in the tribe. However, given that the position of Nunh comes down to who is the strongest male in battle, I could see the position often going to men who wouldn't be good father material. I could see female Miqo'te leaving their tribe in such a situation when faced with the reality of having such a person be their child's father and they're expected to be okay with it... On the flip side, if the Nunh is good father material, then everyone in the tribe benefits from it. While Nunhs aren't often the leaders of their tribes, they do probably have a lot of influence on the next generation they are raising.