Given that studies of sexuality don't 100% declare that heterosexuality is genetic and may in fact be a myriad of different pre-natal, hormonal and genetic flavours, should heterosexuality be considered a legitimate expression of human sexuality?
Heterosexuality is an absurd term, the correct term is normal sexuality.
Replacing a neutral term with a biased term is a cheap rhetorical trick that dodges the issue completely.
First, you're implicitly falling victim to the naturalist fallacy: "it's natural, therefore it's good."
Second, your statement is false. Chimpanzees, bonobos, giraffes, dolphins, buffalo and elephants all engage in homosexual relations (this, by the way, is not even close to an exhaustive list). It is suspected that in many of these cases, male-male homosexual relations help foster the friendships and social ties which are required for a united group or tribe. Other times, homosexual relations are a way for a male to put another male "in his place." An interesting example is rats: when rats are overcrowded, many males will only mate with either males or non-ovulating females. Homosexuality here seems to be a reaction to overpopulation.
Among humans, the Greeks and Romans had a lot of homosexuality for purposes of fostering closeness among soldiers and letting slaves know their place. Native tribes from both Africa and America were found to practice ritualized homosexuality, much to the explorers' disgust.
