Moron's.org have a number of helpful things going on for them. Here is a direct form of response for when you need it in a hurry to respond to that anti-gay ranter.
The canonical list of rebuttals to anti-gay arguments opposing gay marriage
The table of contents is set to easily retrieve the info you need right now. Try it out.
Table of Contents:
Homosexuality is a choice.
Homosexuality is just a behaviour! Behaviours don't have associated rights.
Marriage exists to support sexual reproduction. Gays can't reproduce!
The list goes on and on ... at Morons.org