There's been long-standing traditions of gay marriage, too. So what makes hetero marriage any more sanctimonious?
I'd also point out that the traditions of slavery, religious genocide, and human sacrifices have been in place for thousands of years. Why weaken them just for the sake of change?
Bingo. Two gays getting married in no way, shape, or form does away with the legitimacy of hetero marriage. Nor does it degrade its value.