The U.S. Constitution does not forbid homosexuals from sharing in the promise of equal rights. There for why is it constitutional for any state to forbid homosexuals from sharing the rights of citizens as well as the right to forming legal unions.
The writers of the document never imagined a nation that would discriminate against legal citizens and promised a pursuit if happiness.
Marriage was placed into our legal form until the early 1900's.
In fact NY State law did not recognize the entity of marriage until 1912.
Todays marriage statutes are obviously unconstitutional an there for not valid unless the statutes are changed so that they are made inclusive to represent the intent of the U.S Constitution.