"Whenever there is an adjective added to an important value-based noun, there's an agenda." - Dennis Prager
Social justice is to justice as gay marriage is to marriage. In normal speech, the adjective only modifies the noun; in these instances, it obliterates it.
Aristotle defines justice as giving someone his proper due. It is just to hold a door for a stranger who is trying to escape the rain. But it is also just to punish a robber for his crimes. Social justice, on the other hand, is only a levelling.
The possessions of the rich are an injustice, not because he accumulated wealth in an unjust manner, but simply because he possesses it. He may have made his fortune selling his cure for cancer; he may have made it by being bailed out by taxpayers as the head of an investment bank. To the social justice warrior, the means are unimportant, only the end matters.
Which is to say, justice is unimportant, only equality of wealth matters. Even here, it is unlikely that the social justice warrior will sell his belongings and give them to a less fortunate denizen of the third world. His ire is directed ever upward, never inward.
The matter is similar with gay marriage. Now, marriage has a plain definition: a man and a woman, pledge to be faithful to one another until death parts them. They are, moreover, to care for any children with which they are blessed.
Gay marriage is nothing of the kind. The two men or the two women pretend to take on the role of the husband and wife, but it is a sham display because no children can ever come of such a union. They can only pretend to be married; they can only have children that are not their own.
And, of course, there is simply nothing to prevent other marginalized groups from concocting their own peculiar arrangements which, with the help of an adjective, they can call a marriage. There may be little appetite for three people getting married, but there is nothing illogical about it, once grant gays their definition. And so the affair becomes one uniting any number of people based on mere feelings of affection.
There is some good news: just as the promotion of social justice cannot remove the idea of justice, so the promotion of gay marriage cannot remove the idea of marriage. These ideals exist even if society succeeds in marginalizing them. But it would certainly be better if the State did not work against justice and against marriage.