Brother epan, I believe you may be missing the forest for the trees. The fact of the matter is that homosexual acts are immoral, and legalising homosexual marriage basically enshrines what is immoral as acceptable. It is true that opposition to anti-discrimination laws is opposition to Church teaching, but DOMA is not anti-discrimination. One does not call it anti-discrimination if it is legalising what is immoral. If there was a law that legalised child marriage or polygamy, would you consider it anti-discriminatory? This is precisely the kind of issue we are confronted with.
What matters in the end is what is
moral. We all use discrimination in some way - individuals discriminate between right and wrong; employers discriminate against the hardworking and the lazy; police discriminate between the suspicious and the innocuous; courts discriminate between the innocent and the guilty - however, nobody calls this discriminatory. It is only
unfair and
immoral discrimination that the Church opposes. However, legalising homosexual marriage and destroying the traditional institution of marriage is not moral by any standards of the Christian faith, and it is because of this that we must differentiate between a true marriage between a man and a woman, from a union of two men or two women.
Of course, if your question is why homosexual marriages are immoral to begin with, then that’s a different topic altogether, but I’d be glad to help you understand.