Why gay marriage strengthens traditional marriage.

I was asked to sign a petition recently for the repeal of gay marriage in Washington.  I've always been a supporter of gay marriage just based on equal treatment of the good folks I know personally who are gay, but in the ensuing discussion with my coworker, I realized there's a strong reason for supporting gay marriage if you want to strengthen traditional marriage.

First a question: what is the main threat to traditional marriage?  I'd say the biggest threat is the impression that marriage is not necessarily the normal thing to do when you grow up.  If you want marriage to become more the normal thing, then you need to push examples we see around us from the "not-married" column to the "married" column.  To do that, you need to extend marriage to the approximately 10% of the population who can't have a fulfilling heterosexual marriage.  It's as simple as that!