Voting in the United States was originally established under dozens of assumptions: women should not vote, slavery existed and slaves were only 3/5 of a person for census purposes (and certainly could not vote), property ownership was necessary to be civic, etc.

Tradition, and even legislative intent, are only important when the scope of a law is being reviewed by an appellate court (including the Supreme Court). However, when laws are intentionally changed, that legislative intent and tradition become irrelevant.

From a Constitutional perspective, equal protection under the law has nothing to do with tradition, previous application or anything else. The argument that the law is being applied in a discriminatory manner is the basis for the challenge, and in this case it is clear that there is a valid argument (whether or not ultimately accepted by the court) for discrimination. Prior application can be overturned, even after decades of implementation, such as in Brown v. Board of Education.

Under your reasoning, in the Brown case we would just say, "Well, the laws were written with the intent of having separate schools, and it's been the tradition in the South to keep the races separate all the way back to the foundation of the Republic, and furthermore, the notion that the races should remain separate is a historic reality stretching back to Biblical times, when God enjoined the nation of Israel to remain separate from the peoples around it. We also have Plessy v. Ferguson which decided the matter."

However, the law can change (1) when we affirmatively change it, dispensing with "tradition" or (2) when a successful challenge is made that the Constitutional right of equal protection is invalid.

My response was simply to say that polygamy could be distinguished from gay marriage in a strictly legal manner, because the privilege is for two people. I think that any attempt to normalize three- or four- person relationships (or more) would by necessity fail, because employers would not want to have to extend insurance to more than one spouse, existing inheritance laws would have to be changed, etc. It could theoretically be done, but there is no requirement to change anything to accommodate threesomes.

The point was that there is no "slippery slope".

As an aside, the childbearing argument is a worthless one. There is no requirement that a couple ever have children or even plan on having children in order to get a marriage license. The argument that the homosexual can choose to enjoy the benefits of marriage if only he or she would marry someone of the opposite sex ignores reality, just like a miscegenation law which could theoretically be defended on the grounds that a black person could enjoy the benefits of marriage if only he or she would stay within the African race to marry. The overwhelming evidence that homosexuality is not a choice puts the issue squarely in the same general category as sex or race.