The definition of marriage includes alot more than gender distinctions for Americans. All Americans. Two bodies with interchangeable parts are not ALL that a marriage is based on.
Some 'definitions' have been declared unConstitutional, like the ones where it was illegal for birracial couples to marry too. Were the citizens of VA that objected to Loving vs VA also '2nd class citizens?'
The change is based on equal rights and equal protection under the law. If you can tell me how SSM harms non-SSM marriage in anyway, I'd like to hear it.