Here is an awesome answer (From the Quora website): Same Sex Marriage: Why is having an honest, rational opinion of being against gay marriage considered "homophobia"? Eivind Kjørstad, answered as follows: That happens because there's an increasing fraction of people (including me) who considers that there are no "honest, rational" arguments in favour of restricting marriage to heterosexual couples only. I find a good test for attitudes, is to try the same argument, but exchanging something that's still controversial (gay marriage) for something that used to be, but has now long since been accepted. Here's how this question looks if we, for example, exchange "gay marriage" for equal rights for blacks: Why is having an honest, rational opinion of being against civil rights for blacks considered 'racist' ? Or: Why is having an honest, rational opinion of being against allowing women to vote considered 'sexist' ? Or: Why is having an honest, rational opinion that people who are Jews should not be allowed to hold public office considered unacceptable ? I hope you can see why these things would be considered racist and sexist by many. If you understand why that would be racist, and why that would be sexist, then you also understand the answer to your question.
Commendable! I think arguments like this are the best, and, most likely, the only way to convince people who are against same-sex rights that their arguments don't make sense.