There is no proof that "traditional marriage" is necessary for a polite society and marriage has gone through many changes in the past several hundred years. The founders rebelled against traditions of arranged marriages, based on caste, for romantic love.
The state has no interest in promoting a polite society. It is even prohibited from it in many ways. For instance, the first amendment gives me the right to tell you or anyone else "to suck my balls." It's not very polite but protected by the first amendment.
This notion would make the state's interest so expansive as to be totalitarian, which is exactly what social conservatives are.
The discrimination is based on what they do in private. I did not say it prevents what they can do in private.
Actually traditional marriage is necessary for a polite society. The non-traditional examples that you gave are examples that our society has rejected.
Marriage is not a private issue; it is an activity licensed by state governments.