First, a man and a woman fit together physically. We see this most clearly in the outlines of the genitalia of a male and a female. This is a fitting that obviously is not present with two men or two women. Moreover, sexual intercourse involves precisely one man and one woman. The human bodies of the man and the woman therefore point to monogamy and sexual exclusivity—and those of same sex couples point to abstinence from sexual activity altogether.
Excerpt from “The High Cost of Denying the Obvious, Part 3”: Biological Differences Between the Sexes