Again, this whole "biological root" language is misleading. It is clear that biology does not have "primacy" in the development of sexual orientation; if it did, identical twins--again, genetic clones!--would have very high concordance rates for sexual orientation. They don't. The evidence clearly supports the claim I've been making ad nauseam: it is just as wrong to claim that the primary influence on behavior is biology as it is to claim that the primary influence on behavior is the environment. The quicker we get past this goofy nature/nurture debate, the better.
Miles
^^^^ CB: The biological is "primary" in time, in that one gets one's genes before one gets one's culture.
So, how does this "complexity" work ? There's this biological influence and there's this environmental influence, neither being "primary", which seems to mean neither is predominant or determinative over the other.
So, person X is born with an influence from biology to be attracted to the same sex or to the opposite sex or both. This would constitute the instinct of the individual. Then they get an influence from culture such that they are influenced one way or the other. Then , the individual turns out attracted to one , the other or both, based on the resultant "summation" or complex calculus of these factors.
Seems to me that by that complexity model, the answer is that some people do have heterosexual instinct. Their instinct arises independently of their enculturation. Their enculturation may reinforce or contradict their instinct, but they do have an instinct. The fact that their instinct isn't the only factor in determining their behaviors doesn't mean that instinct is not _a_ factor in determining their behavior.