One of the most persistent and off-putting questions in the same-sex marriage debate is "How does it (same-sex marriage) hurt you?"
Traditional marriage defenders usually find themselves scrambling for some sane answer but usually end up stumbling, and for good reason. Marriage is so fundamental that its nature has never been questioned...until now. In short, it has never required an explanation or a defense, thus most don't have one.
The best answer to the question is the return question: "How do you know it won't?" For the real answer is we don't know. Sociological change may take a generation or two before there is measurable impact on the larger society.
Meanwhile, the gay agenda folks are very self-assured with their "how does it hurt you" question, and putting the burden of proof back on them is something they are not ready for.
Here's an article on the subject: "Yes, Marriage will change--and here's how"