I've wondered that myself. I suppose the answer lies somewhere in the area of what makes it so important that they be "married," as opposed to joined by civil union. They don't want any asterisks in the playbook.
It's the Orwellian, leftist supported technique of controlling language to control conceptualization of difference.
It’s all designed to shove the “homo-normative” delusion upon us. They want to force us to embrace queerdom.