possible, but not natural
So you oppose artificial insemination or other types of fertility treatments for childless couples seeking to have a baby? It's not "natural" after all. And drugs to treat illnesses. Those are often synthetic, and not "natural", so we can't administer them to the sick?
The whole "natural" argument is a losing one and problematic. "Marriage" is a loaded term, so I prefer something like "Civil Union" and it can be defined legally according to however the state (as represented by the people) wants to. The word "natural" would not be a part of that definition.