I don't believe marriage was instituted to protect the rights of the woman or the man but the offspring of such a union. If procreation or adoption is contrived by a homosexual couple that is simply a contractual issue and both can be facilitated by a civil union.
It is the "Right" of a child who is born into this world to be protected and nurtured by those responsible for its existence. The word marriage has been articulated throughout history to mean exactly that.
To try to pervert the term to include homosexual unions diminishes the rights of a child. It does not enhance the union of homosexuals. It endangers the sanctity of a heterosexual union and diminishes the moral value of a child by delegating it to a contractual obligation which can be legally dissolved.