Researchers at the Karolinska Institute found that variations in a gene for vasopressin correlate with monogamous tendencies. Vasopressin seems to influence some bonding behaviors in males; it's influenced by testosterone; and it's also responsible for defensive and aggressive behaviors, often in defense of the mate and family.
According to Bloomberg Muse:
The researchers ran genetic tests on 2,186 participants in the Twin and Offspring Study in Sweden and had them fill out a survey about the quality of their marriage. Men with a genetic variation scored significantly lower on a scale of partner bonding. One in three reported a crisis in their marriage within the past year, twice the number as those without the variation.
... Those whose husbands had one or two copies of the gene variation scored significantly lower on tests asking about their marriage quality than those without it.
Larry Young and Elizbeth Hammock of Emory University had found that variations in parts of the vasopressin gene, formerly considered "junk DNA", seemed to influence monogamy in the prairie vole. While this rodent is socially monogam0us, some individuals never mate. Hammock was able to breed non-monogamous prairie voles by selecting for this variation.
I heard Young present about this at a conference, and he was very cautious about extrapolating his vole work to humans. Paul Lichtenstein, a professor of genetic epidemiology at Karolinska, has kindly connected the dots for us humans.