They simply Refuse to Acknowledge their Natural Design, either by Choice or Defect, and Society doesn't have to Cater to that and Change the Inherent Definition of Marriage for it.
I'm pretty sure society does not have to do so.
But they will.
And just like today, when people look back at the civil rights movement in the South 50 years ago and wonder what the fuss was all about (and think poorly of the people who tried to stop it), 50 years from now, when gay marriage is widely accepted, people will also look back at this time and wonder what all the fuss was about.
You are on the wrong side of history if you oppose gay marriage today.