The petition can only be signed by those over 16 years of age. The issue is not to do with a lack of tolerance of homosexuality. As Mishap so rightly said, plenty of Catholics would have no truck with that sort of bigotry.
The issue for the Catholic church, and many others, is the redefinition of marriage. If society accepts this change, then will society be willing to accept marriage between multiple partners, marriage to animals, marriage to objects (not far fetched, have actually been claimed to have happened). If not, why not?
I know I may be old fashioned but for me marriage is between a man and a woman. I have no problems with same sex relationships, partnerships with loving individuals with the respect of society and the full backing of the law, but why can't it be something equally valued but different from 'marriage'?