My dad pointed out that in Mexico, church and state are completely separated so you go to the courtroom to get your civil union, and then you go to the church to get married. And he suggested that there are other kinds of couples that might want to apply for tax benefits - for example, unmarried sisters who decide to live together into their old age. He suggested that we could have a notion of "household" which didn't require any particular format, and this way we could eliminate the regulation of what kinds of relationships or commitments are recognized and not recognized.
My mom's 2 cents were that the moral status of marriage should be preserved, and that we could have civil unions for gay people, but that the distinction between marriage and civil unions shouldn't be based on sexual preference of the people in the relationships, but on whether or not the couple had children. So a childless heterosexual couple would have a civil union, but a gay couple with an adopted child would have a marriage. She thought this made sense both because children are the real reason tax benefits are awarded for marriages, and also because for the chilren's sake, she wanted to enact rules that made it a lot harder to get divorced from a marriage than from just a civil union.
My brother-in-law came in at the end of the debate and argued that if gay marriage became legal, sooo many people would take advantage of it by getting fraudulent marriages. He said he knew so many people from the marines who would do it in a second. Like, if two friends were living together and one of them had a good job but the other one just couldn't find employment. My answer to that was that well, we shouldn't live in a society where the taxes are bad enough and the incentives great enough to want to go through the trouble and deception that entails. He conceded the point, but well, it's pretty moot.