Once you change the definition of marriage, you lose the right to say no one else can redefine it. So what form will it take next? Will I one day be able to marry my house plant and my TV for the government benefits? It’s no less legitimate than men marrying other men for the supposed big brother benefits.
And lawyers. Can you imagine how many more divorces they will benefit from.
One equally valid and peaceful culture already has "marriages" for prostitution. I imagine we'll have marriages for gun transfers if private sales are regulated, for tax purposes (a New Year's Eve marriage for a day to transfer paper losses to someone with high taxable income), to avoid inheritance taxes (marry your father on his death bed), and similar trivial abuses of what used to be a sacred covenant.