Well I can only speak from experience. It is a preference but it's a preference in the same vein as the physical itching need for some people to climb mountains, or jump from airplanes or a general need for an adrenaline rush while others are naturally phlegmatic and cannot stand loud noises, for example. It does have to do with the chemical balance in your specific body and the wiring of your specific brain. Now this can be fostered or kept in check somehow but the predisposition is real (whether genetic or due to exposure in early childhood is difficult to say). Which is why even if we get rid of the nuclear family as an ideological standard you will still see many, maybe even overwhelmingly most, people practice exclusive monogamy.