Abstract
The New Natural Law Theory (NNLT) argues against the morality (and legality) of same sex-unions on the basis that homosexual (and non-reproductive heterosexual) acts are unnatural, unreasonable, and therefore immoral. In this paper, we explore and critique the foundational principles - biological and personal complementarity, their subcategories, and the interrelationship between them - that the NNLT uses to justify its claim. We propose alternative principles - orientation, personal, and genital-biological complementarity, with a distinct interrelationship - to argue that homosexual couples can engage in sexual acts that are natural, reasonable, and therefore moral. Our study clearly demonstrates that for the NNLT genital complementarity, a subcategory of biological complementarity, is the sine qua non for personal complementarity. In other words, personal complementarity within a sexual act is only possible if there is genital complementarity between male and female. We believe that the NNLT's foundational principles reflect too narrow an understanding of the human person and human sexuality. Instead, we propose "holistic complementarity" as the fully human integration of orientation, personal, and genital-biological complementarity. What defines a natural, reasonable, and moral sexual act is not genital complementarity as the foundational principle, but a dialectic between these three principles of complementarity.