This is essentially a comment I posted in response to a comment on this post over at the Touchstone Magazine blog, Mere Comments.
I was responding to an argument which stated that any kind of natural law argument against homosexual "marriage" is bound to fail, since people who make such arguments do not refuse to acknowledge marriages between couples who are infertile due to certain medical conditions or to age.
I responded as such:
[This argument is] False.
The definition of marriage does not include the essence of each particular conjugal act, but rather the essence of the conjugal act in view of the marital union as a whole. The union between a man and a woman is essentially fertile. That some particular privation may render some particular conjugal acts infertile does not change this any more than the roof of my house changes the essence of the helium in a balloon simply because it gets in the way of the helium finding its natural level.
Homosexual acts are essentially infertile rather than being rendered infertile by some privation. It is impossible for two men or two women to produce children. Homosexual acts are different in kind from the conjugal act, and thus homosexual unions are different in kind from the marital union. That the law should recognize this is a matter of justice, since justice involves treating similar things similarly and different things differently.