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.

This is possible, though I would note that sex is just one of the things one gets from a romantic relationship. Even if a poly society would leave more men without sex, it might provide more men with things such as close companionship. It is not obvious which one is more important. (Companionship is far more important than sex for me, though I'm probably atypical for a male in that regard.)