If the captain of a football team has five women, that means that he only has one-fifth as much time for each of them, meaning that they're likely to be available to others as well.

At first sight it seems that those women are 4/5 available for other men in the group. But this assumes that men and women have the same sex drive on average. If we assume that men have stronger sex drive, or that their sex drive increases significantly when many women are sexually available to them (I am not a biologist, but I think both of this is true), then there is less than 4/5 availability of these women for the rest of group.

In other words, to make all members of a poly society equally sexually satisfied, this society must have more women than men. With the same number of women than men, less successful men will be frustrated, even if all women are satisfied. (Of course, if you are a woman, or if you are the most attractive man in your poly group, this is not your problem.)

EDIT: In essence, "one fifth of time" does not equal "one fifth of sex". A woman may spend one fifth of her time having hot sex with the captain, and the remaining time in just-friends mode, or 90% just-friends mode, with the remaining men.

And perhaphs, since they're getting their desire for high-status übermasculinity satisfied from him, they'll also be more open to relationships with less masculine and lower-status men.

Or perhaps, their demands will increase, and the remaining men will seem even more pathetic.

It seems to me that for most men monogamy is better. For women, two topics to think about: children and age above 40.

When the children are born, do you want to test paternity or not? (But even if you won't, some man will think that he is a father, and the others will think they are not. Or maybe, everyone will think that someone else was the father.) It seems like most men do not want to invest much resources into child that is not biologically theirs. Even if the man has one child with one woman, and three children with other woman, he may invest little into the first child.

If you are a young woman, it is important to note that the balance in "sex market" depends on the age. On average, younger women have higher sexual value than younger men, but older women have lower sexual value than older women. Thus we have so many young boys unable to find a girlfriend, and so many old women unable to find a partner (this imbalance is even worse because women on average live longer). Don't assume that your "sex market" value will stay constant.

Both monogamy and polygamy have their benefits and risks. The risks of monogamy are well known, therefore I wrote about the risks of polygamy. (Risks of monogamy: choosing the wrong partner and not having enough data to realize it; also if your partner dies or leaves you, you start from zero.)