Expanding the definition of marriage

As is discussed in the linked video, I have thought for some time that the push to recognize and legalize gay marriage in the U.S. will be a wedge event to drive acceptance of other non-traditional forms of marriage, marriage with multiple partners (i.e. group marriage). My thinking on this goes as follows:

• The original marriage construct was meant for the creation and raising of children.
• The government got involved because it was in their interests to promote families.
• Gay marriage does not have as its base, the creation and raising of children, it is for the couple, the relationship. The desire for legal recognition which will drive social services and have legal ramifications.
• If gay marriage is accepted (a relationship between two people of the same sex that will not/can not produce children) as a number of states have or are trying to do, why not three men? What is the legal difference between a marriage involving 2 men and one involving 3? What would be the bases for denying the rights of three men to get married if it is legal for 2 men to get married?
• If three men can get married, what would be the legal basis for denying two men and one woman from getting married? The precedent for multiple partners has been set within the same-sex construct.

I am not saying I am for or against any of this, I am just saying that I think that this is inevitable based on what is happening to the definition and legal description of where marriage is going today. I think that the major push-back will come from government agencies where the multiple partner families start demanding government services like SS and Medicare/Medicaid. This will also play havoc with the definition of a legal parent. Is it the biological parents only or all partners in the marriage?

This idea has been slightly explored in a Robert A. Heinlein novel "The moon is a harsh mistress".

If my logic is in error, please comment.