Marriage, as regulated by Govt as opposed to as performed by a Church, is a form of contract. If a State govt sets the terms of contract, regulates the various and sundry benefits accruing therefrom, and regulates the process and terms for arbitration of breech or dissolution, then under the 14th amendment, no State shall make any laws that do not provide equal protection to all citizens and resident aliens. The State regulates marriage between couples that are unable to have children and does not require childbirth as a condition of the agreement. The State does not prohibit childbirth outside the agreement and in some jurisdictions determines couples to have entered into an agreement even if they have not sought State sanction (common law marriage). It is impossible for me to read the 14th Amendment without understanding that if the State regulates marriage, then it must do so for any two parties wishing to enter into it. Unfortunately, I also find it hard to understand how polygamy can be prohibited provided equity between parties to the agreement could be established.
"No State shall make or enforce any law which shall...deny to any person within its jurisdiction the equal protection of the laws." How we as Second Amendment advocates can find that any harder to understand than "...the right of the people to keep and bear arms shall not be infringed" I do not understand.
I don't have to like or approve of gay marriage. Others do not have to like or approve of private ownership of AR-15s with 30 round magazines. But until the 14th and/or Second Amendments are themselves legally altered or nullified, both are rights assured by the Constitution.