Gays, unlike polygamists, can be married in a civil ceremony in any state in the union. The question is can does the government then have to recognize said private ceremony. Further in many states if you wish to marry your first cousin or closer you can be arrested. Genetic preference discrimination? Does having different marriage ages in different states constitute age discrimination?
Wait, are you justifying the discrimination against gays by saying that other kinds of discrimination exist? You can't actually justify the categorization that states have created, so you're just going to flatly defend discrimination in general?
Unlike gay discrimination, discrimination based on age or blood relation (not family relation) actually does have a justification:
Young ppl cannot consent to sex or any serious contracts under our legal system. And no, different ages for different states is not discrimination, that has nothing to do with discrimination, that's different rules in different states. Discrimination is where a state applies the rules differently (or applies different rules) to different ppl within their jurisdiction. The fact that two separate entities make two separate decisions on the same issue is not discrimination, its two different entities.
Banning marriage based on blood relation (or at least banning intercourse) has a basis, and this was the rational, secular reason for our laws against incest. Centuries ago, adoption was almost nonexist, and so incest really did imply blood relation. Today, obviously that's not the case, and there is no justification for a category where ppl can't married or have sex if they have no blood relation.
But unlike banning child marriages or incestuous marries,
there is no justification at all for banning gay marriage, hence you cannot articulate one. And if you could articulate one, I suggest you go to law school and become a lawyer, because you'd become the wealthiest attorney of all time. The argument doesn't exist, federal judges laugh out loud and say "You just can't come up with it, can you?," its like alchemy. Its not happening.