I'll expand. In a monogamous society, the alphas still get the pick of the best women for procreation. The further down the totem pole for men, the fewer their choices. But they still end up with someone and the ability to procreate. Only the truly undesirables get left out and we don't want them in the society anyway. Additionally, marriage and the family unit is designed (prior to welfare) to force a man to produce enough to pay for his own reproduction. Thus, the men with the most valuable and desired traits ina society will then tend to be more successful financially and be able to afford many children. The less capable a man, the less procreation he can afford. If he has too many kids, some starve if he is unable to care for them. Thus, a monogamous society strikes a very good balance between eugenics and mass labor participation. The best men still get best pick of women and can afford to produce more offspring. The less desirable men produce less offspring, but they are still able to reproduce which buys their participation in the advancement of society. Over time, the best reproducing at higher rates still produces a eugenic trend in society increasing the quality of the gene pool. In addition, the society advances more quickly because it is more likely able to produce a surplus to fund specialization and it has more manpower for defense.
Monogamy/marriage is a powerhouse of a tool for building advanced societies.