One could also say that sex itself is not biological, since there are many out there who choose or are naturally inclined to celibacy.
I contend there is a strong biological connection to our behavior, both mono and poly.
My "Survival of the Horniest" Theory:
Members of a sexual species that do not procreate do not pass their genetic information to the next generation.
Members who choose partners for procreation discriminately increase the odds of their genes being passed on successfully in not only the next generation, but following generations (Strong, smart, stable, &c = attractive)
Some of the traits passed on are things like sexual drive/aggression, health of sexual function and personality traits which help progeny "get laid".
Over time, the "stronger" tend to breed more and the less desirous of sex breed less. The population reflects these variables.
This is my "Survival of the Horniest" Theory.
IMO, monogamy is a tendency (biologically) to play conservatively, staking all resources and energy on a single most viable partner.
Polyamory is less "all the eggs in one basket" and spreads resources over a wider field. This is a better strategy for males of our species (in terms of procreation) as one male can impregnate droves of females whereas females are limited by the time constraints of gestation to a handful of breeding partners.
I do not think mono or poly is entirely biological, but I think there is a strong connection. Part of being a human is the ability to override a natural genetic inclination (instinct) and make choices based on logic, right down to denying the very basic instinct of self-preservation in some situations.
Often he had been often bound with fetters and chains, and the chains had been rent asunder by him, and the fetters broken in pieces: neither could any man tame him...and he asked him, What is thy name? And he answered, saying, My name is Legion: for we are many.