... or past experiences, or both 

https://www.forbes.com/sites/zhanavrangalova/2017/09/23/monogamous-and-nonmonogamous-mens-brains-respond-differently-to-romantic-images/#611af63b7166

There is quite a bit of diversity in people’s tendencies toward (non)monogamy, from those perfectly satisfied with one lifelong relationship, to those who seek—and sometimes obtain—hundreds or thousands of partners.

What kind of brain differences might these behavioral differences reflect? Would the brains of nonmonogamous people process romance-related information differently from the brains of monogamous people? A new fMRI study suggests so.

Researchers at the University of Texas at Austin brought 20 sexually active heterosexual men (aged 22-50, average = 34) into the lab, laid them down in an fMRI scanner and showed them different images, including sexual (e.g., naked people having vaginal intercourse), romantic (e.g. fully clothed people hugging or hand holding) or neutral (e.g., landscapes or people barbecuing).

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Results showed no differences in brain activation between the monogamous and nonmonogamous men when they were watching sexual images: Both groups found these images equally arousing.

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However, the brains of the two groups differed quite a bit when it came to romantic stimuli. 

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Furthermore, among monogamous men, areas that were active when viewing romantic pictures were still active while viewing sexual pictures, indicating that sexual and romantic stimuli are closely related for them. By contrast, nonmonogamous men’s brain showed additional activity during romantic stimuli in several regions of the cortex that were not active during the sexual photos (snip) This suggests there is a greater difference between romance and sex for nonmonogamous men. As Lisa Dawn Hamilton, the lead researcher in the study explains, “It seems nonmonogamous men are processing these images at a higher cognitive level instead of responding mostly with the ‘reptile brain’ (as Dan Savage would say). It’s as though they are studying a piece of art rather than something that is intrinsically rewarding to them.

Past studies have found that people in polyamorous relationships have higher testosterone than people in monogamous relationships, and that the sexually adventurous (like those more likely to have casual sex, to have cheated on a partner or to have interest in group sex) are more likely to have a certain type of dopamine-receptor gene. This new study adds yet another piece of evidence for the biological underpinnings of people’s tendency for monogamy versus nonmonogamy.

Of course, finding differences in brain activation doesn’t necessarily mean that these men have a biological predisposition toward (non)monogamy driving their behavior. It could be that men who are monogamous have had more rewarding relationships in the past and thus conditioned to associate romance with pleasure. Or it could be a combination of the two: Some brain differences were already present at birth, and later relationship experiences only amplified those initial brain differences.

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