Imagine, if you will, that somebody establishes the Church of Gay People, which is entirely legal under our Freedom of Religion. The CGP institutes for itself a sacrament of marriage, which is also legal because nobody owns those words. Their marriage, of course, is only between two men or two women. So far, this probably falls under most people's idea of "none of my business".
Now here's where the CGP gets frisky. It begins to lobby the government to make its definition the only legal one! Heterosexual marriages would no longer be recognized if they are successful. Now, straight people don't really need to worry, because we probably outnumber gay people greatly. However, the point is that the CGP's claim is logically no stronger nor weaker than that of any other religion, differing only in the number of believers. Why should the government be in the business of deciding which religion's marriage is the proper one? Put another way, if you don't want your definition of marriage to ever be nullified by government (which you probably already do not trust, given the demographics), you shouldn't oppose a gay person's definition of it either.
The sanctity of your marriage stems from your love and faith, not in the least bit because a government bureaucrat stamped your paper. In fact, I dare say that even if the government got entirely out of the business of marriage (strike "married" from the tax forms, etc), your marriage could still be sacred and happy.
Which means what the opponents of gay marriage are trying to do is to use the government as a tool to force non-believers to conform to their religious views. Stop that.