"f a person was born on a deserted tropical island & unfortunately lost both parents but survived to adulthood alone with no other human contact would he/she know if they were homosexual or heterosexual, & most of all would it matter ?"
This is a really interesting question. I think it's worth noting that we didn't really have the language of 'homosexuality' or 'heterosexuality' before the nineteenth century.
Prior to this, the concept of a homosexual as a category of person who seeks romantic/sexual relationships just with people of the same gender didn't really exist. People spoke of 'sodomy/sodomites' - but, like all sins, this was something that anyone who became sufficiently corrupted was considered capable of. Athenian society largely seems to have understood sexual relations in terms of 'dominant' and 'subordinate' - the actual sex of the subordinate was unimportant, just so long as the 'dominant' partner was of superior social standing and the subordinate one was of inferior social standing (i.e. a woman or a slave).
These aren't concepts that a human being whose only contact with others had been his (dead) parents would have access to, and the only understanding of sex would be that of animals. Others have noted the strong biological/genetic component in sexual attraction which seems to be evident, but I think the person would have no real concept of sex as we understand it. If he were rescued, he would likely be diagnosed with numerous developmental disorders.