I think there is no question of "biological predisposition". However I think it's even more important to question the concept of "biological predetermination" !
Big difference there.
The poly vs mono debate is really no different than debates about a number of medical nature/nurture debates. Cancer might make one good example.
But recent research is starting to confirm what seemed to be known by the ancients. That we have much more control over the whole DNA/RNA process than the original "predeterminists" may have wanted to acknowledge.
Having been something I've been forced to study and follow for a number of years I'm maybe much more open to this concept.
I wish I had specific links close at hand that I could post that might be helpful to those curious enough to follow up on, but the only one that springs to mind right off might be research and a recent book called "The Biology of Belief" - Bruce Lipton PHd. I think this was a decent attempt at explaining complex science in layman's terms and I'd recommend it for anyone just getting their feet wet in the nature/nurture debate.
The whole debate about DNA sequencing etc has profound implications for classic western medicine, especially in the realm of brain function & chemistry. I'm sure most are aware of all the horror stories surfacing every day about the ineffectiveness and danger of trying to manipulate brain chemistry via drugs as a long term or permanent strategy. It simply doesn't work (long term).
Sorry for rambling............
My point being that "bilogical predestination" is very much in question. How that would apply to a mono/poly debate is up to the individuals. It would SEEM that we have the capabilities to shift ourselves in any direction that we truly believe would be a beneficial shift. If we see no benefit, no shift will occur.