That some acts are intrinsically evil, and that by condoning them, we as a society condone that evil.
Evil acts make evil people; even if the acts themselves are carried out in private, the people who commit them live lives that are public, and thus carry the evil into society until it becomes woven into the culture itself.
I do not wish to live in a culture where evil is condoned; therefore, I oppose the evil of sodomy, even when carried out in private between consenting adults.
Sorry. I was referring to the legal argument. The moral argument is fairly simple - but was not only discounted but dealt with in a contrarian manner by the court.