He said that gays have the right to marry, just like everyone else - everyone is allowed to marry a member of the opposite sex.
This argument was shot down - quote from Warren's decision in Loving v. Virginia.
Most of the remaining time was spent arguing that homosexuals shouldn't adopt children, because the children raised in homosexual families are apparently more likely to not do well in school.
The quality of this research was attacked (especially by the professor from the Child Development department here on campus). Not only this, but I made the argument that kids waiting for adoption are far worse off than a child in any stable family environment, homosexual or heterosexual.
He tried to say that it was going to cause a moral decline of society, but claimed he was agnostic. He admitted this was a slippery slope argument.
I pointed out that this was a logical fallacy - an attempted proof by induction without meeting the qualifications of one. He said "Oh, it's a weak argument, but I don't think it's a logical fallacy." --
http://en.wikipedia.org/wiki/Slippery_slopeHe also made some funny claims - claimed that I needed to read a statistics book, whereupon I gave him an education about correlation, causality, and lurking variables.
Also, just for good measure, had a quote from the VA Atty. General in the Loving v. Virginia case in 1967, claiming that declaring the ban on interracial marriage unconstitutional would lead to polygamous marriage and incestuous marriage, and then asked him where these things were happening.
I think that covered most of the bases. If you want to know if he rattled off any of the talking points, ask - I may not remember offhand, but if someone says something similar to what he said, I'll remember.