Human brains have three layers of programming. Each layer adds a twist or turn to sexual preferences and tendencies. The first layer is genetic progamming from the inherited genes. The second involves environmental influences that impact genes and their expression. The third level deals with the way we "fill in the blanks" as social and cultural beings. This level can become a feedback loop that influences the inherited genes by influencing with whom we choose to have sex.

Darrel Ray, Sex & God: How Religion Distorts Sexuality