In male rodents (voles), differences in the gene for the promoter for the vasopressin receptor, determines monogamy or polygamy. Interestingly, the same effect is also similar in humans. Human males are more likely to be in committed stable relationships if they have one of the two promoters. Why is this not on 23andMe?

Genetic variation in the vasopressin receptor 1a gene (AVPR1A) associates with pair-bonding behavior in humans
The Oxytocin–Vasopressin Pathway in the Context of Love and Fear
Oxytocin and vasopressin receptor polymorphisms interact with circulating neuropeptides to predict human emotional reactions to stress
Impact of Childhood Adversity and Vasopressin receptor 1a Variation on Social Interaction in Adulthood: A Cross-Sectional Study
The AVPR1A gene and substance use disorders: association, replication, and functional evidence

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