Well, first of all the Founding Fathers -all of them- were dead before the ratification of the Fourteenth Amendment, so your reliance on them for the question of the constitutionality of gay marriage is clearly misplaced.

Eventually, laws forbidding homosexuals the right to marry (a fundamental freedom) will be challenged on the basis of the Due Process and Equal Protection Clauses and probably the Ninth Amendment. It wasn't until 1967 when the Court recognized a constitutional right for biracial marriages. Opponents of the court's ruling used similar insipid arguments that there is nothing in the constitution allowing such an unnatural union and that it would bring about the ruination of civilization.

The right to privacy was first protected as a constitutional right in a case called Griswold v. Connecticut, in which the Supreme court found that a state law that made contraception was a crime. It was classified as a penumbral right as it originated from the I, III, IV, V amendments, which embody privacy values. The right was also viewed as being part of the IX Amendment, which recognizes that the rights, enumerated in the bill of Rights, are not an exhaustive list of protected rights.

Also, the Due Process Clause of the XIV Amendment prevents legislative efforts to deny the people of life, liberty and property. The concept of liberty within this context has been interpreted as encompassing individual privacy rights. This was the basis for Roe v. Wade, and while I may disagree with the outcome of the decision, I feel the application of the privacy right was appropriate for the test of the constitutionality of abortion.

The privacy right, found in the Fourteenth Amendment, was the basis for declaring statutes criminalizing sodomy as unconstitutional ten years ago.

Excuse me were you there in 1787? You don't know how the founding fathers thought or what their intents and purposes were