The best way to answer this question is to point out that it misrepresents the important issues. The whole metaphor that instinct or biology is a core "underneath" the veneer of culture/symbol/social construction was discarded years ago by anyone who seriously studies human behavior. As any competent population geneticist will remind us, the characteristics displayed in a species are produced by the complex interplay of natural selection (at both the group and individual levels), interactions within and among species, existing environmental conditions, and calamitous events. There is no biological "instinct" beneath it all; behavioral patterns are produced by complex interactions
among all these factors.
So I propose that we replace the "veneer" metaphor with a "weaving" metaphor (inspired by Kel, I guess!): patterns of behavior are woven from diverse strands of biological, cultural, psychological, and environmental "yarn". Just as it is silly to ask "which of these strands of yarn are at the core of this rug?", so is it silly to ask "underneath a behavioral pattern, is there some pure instinct?" (I know Justin's been making this point on the list for years, but perhaps my quaint metaphor will help.)
One thing I dislike about the "weaving" metaphor: you can take the metaphor too far and say there's a Weaver at the loom, producing the fabric; other than that, I think it's helpful.
So the short answer to CB's question:
"Underneath" all that culture that Butler and others discuss, do any significant number of humans have instinctive attraction to the opposite sex or is all sexual attraction socially constructed/cultural/symbolic ?
Yes, definitely!
Miles