The argument over the denial of rights based on sex is an interesting one. In federal jurisprudence we never amended the constitution to provide "equal rights" regardless of sex (the ERA). This might be the only thing preventing same-sex marriage now.
Because it is clear that if a PERSON can marry a woman, the ERA would have prohibited discrimination based on the SEX of the person that could marry a woman.
Since we didn't pass the ERA, we are still able to discriminate between sexes.
Note that the current law does not in any way discriminate against gay and lesbian people. Every person, regardless of their sexual preferences, is free to marry any person of the opposite sex.
On the other hand, the constitution does not give anybody the constitutional right to marry the person they are attracted to (otherwise we would have to allow polygamy and half the population of this board would apparently be married to Ann Coulter :->).
"Since we didn't pass the ERA, we are still able to discriminate between sexes. "
But a lot of states have state ERAs in their constitution, and those are being used to argue for gay marriage.