To date, biological explanations of sexual orientation have broadly focused on genes and/or prenatal hormonal environments which are thought to act on the brain to provide the neural circuitry on which sexual orientation is inscribed. The proposed models are open to criticism when applied to a healthy population at large because of the implied reference to developmental anomalies. For this reason the present paper challenges the traditional viewpoint and hypothesizes that the foundation of adult sexual orientation may be the result of adaptive programming beginning before conception. According to this hypothesis the continuum spanning human sexuality has its etiology defined in terms of male and female-mediated forms of selective preconceptual marking. The hypothesis also assumes that, via the mechanism of genomic imprinting, the imprinted gene is able to switch through different states of potential activity from the incomplete to the fully penetrant state resulting in a continuum of orientations ranging from asexual, through graded bisexual to homosexual. An adaptive preconceptual program has biological significance as it ensures a generational preparedness for the prevailing conditions. The physiological aspects and circumstantial evidences which were important in developing the new hypothesis are also described.