Do you agree with the Kinsey scale of human sexuality?

[jaycee]

I'll just offer some evolutionary biology I'm familiar with. Of course, being a physics student, I'm not a specialist, so I'll avoid conjecture where possible.
Obviously the purpose of sexual attraction is that it leads to procreation. After all, animals without desire to procreate won't produce offspring, thus removing asexuality from the gene pool (although that's not to say it never appears, as many asexual humans will testify... anyway, perhaps genes promoting asexuality reappear through a mutation?)
Homosexuality is an occurrence of genes "misfiring" - genes promoting a behaviour that won't actually promote the survival of the individual's genes. So in that sense it should be clear cut heterosexuality or homosexuality, as you (presumably) won't have genes that promote behaviour somewhere in between (although there could perhaps be genes that basically say "be attracted to any member of your species").
Now, for nature vs. nurture... the best way to understand this is to say that genes "react" to the environment they are in. Or, put another way, some genes "turn on" and others "turn off" depending on how we're treated and what we experience while we're growing up and well into our adult life. So ultimately "nature" puts all the bits in place, and "nuture" influences which bits are used and which aren't (needless to say the majority of bits are used).
As I said in my brief disclaimer, I'm not a biologist, so I'm not the best person to ask. The best options are to read the likes of Richard Dawkins or ask other evolutionary biologists, particularly sexual selectionists (i.e. specialists in things such as gender roles in animals).
The most important thing to remember is that all variation ultimately comes back to genes (I mean, the existence of life must necessarily come before any culture, and it's the genetic variation across the various geographic regions that gives rise to said culture), whether or not they are influenced by their environment.
Anyway, I hope what little I said was at least somewhat insightful...