According to
Dictionary.com, there are six definitions of marriage. Of those, one adopts the definition supported by any-gay marriage proponents (being dual-dender specific). The other five definitions suit the argument of free-thinkers (gender-neutral).
1.
a. the social institution under which a man and woman establish their decision to live as husband and wife by legal commitments, religious ceremonies, etc. Antonyms: separation.
b.a similar institution involving partners of the same gender: gay marriage. Antonyms: separation.
2.
the state, condition, or relationship of being married; wedlock: a happy marriage. Synonyms: matrimony. Antonyms: single life, bachelorhood, spinsterhood, singleness; separation.
3.
the legal or religious ceremony that formalizes the decision of two people to live as a married couple, including the accompanying social festivities: to officiate at a marriage. Synonyms: nuptials, marriage ceremony, wedding. Antonyms: divorce, annulment.
4.
a relationship in which two people have pledged themselves to each other in the manner of a husband and wife, without legal sanction: trial marriage.
5.
any close or intimate association or union: the marriage of words and music in a hit song. Synonyms: blend, merger, unity, oneness; alliance, confederation. Antonyms: separation, division, disunion, schism.