Dear PV,
If marriage between US citizens is protected by the 14th amendment and can't be abridged without violating the constitution, how could any marriage be prohibited? If marriage between two men or two women can't be prohibited based on the 14th amendment, how could marriage between more than two people or between an adult and a child be prohibited given they are all US citizens?
There are probably other arguments for and against gay marriage (both civil and religious...do you really want the government regulating religious ceremonies within a church?), but don't think this argument applies as marriage isn't a specific privilege or immunity defined in the constitution, such as voting. It may be covered by the last sentence in the 14th amendment on equal protection, but that will be for the courts to decide and a state law has to be passed for it to be challenged through the courts.
The fact that 38 states, the number needed to ratify a US constitutional amendment, already have a ban on gay marriage means if the US Supreme Court rejected a state ban based on the 14th amendment equal protection clause, a constitutional amendment would rapidly be approved.
Barry Trout