What's Wrong With Polygamy?

Charles Krauthammer notes that polygamy is just around the corner.

To simplify the logic, take out the complicating factor of gender mixing. Posit a union of, say, three gay women all deeply devoted to each other. On what grounds would gay activists dismiss their union as mere activity rather than authentic love and self-expression? On what grounds do they insist upon the traditional, arbitrary and exclusionary number of two?