That's an interesting point. Brings up the question of what exactly is heterosexuality other than the obvious biological aspects. I think it's been established that sexuality in terms of procreation is basically ruled by chemicals and the instinct to perpetuate the species. After that... who knows. If robots/androids were perfected to a point that surgery or imaging machines would be required tell distinguish a human from a machine there would be questions like yours. If there was an android in female form that you were attracted to and a relationship developed... would that mean you were no longer heterosexual? I wonder.