Marriage menace???

More unreasonable rubbish is today being spoken against gay marriage with this: The Roman Catholic Church in England and Wales is stepping up its campaign against the government's plan to legalise same-sex marriage. In a letter being read in thousands of parish churches, the two most senior archbishops say the change would reduce the significance of marriage, and that Catholics have a duty to make sure it doesn't happen. Archbishop Peter Smith told the BBC a married couple and their children "forms the basic building block of a healthy society". (more here and here)
Why on Earth would allowing gay men and women to get married be any threat to heterosexual marriage? How would it or could it hinder the Catholic view of marriage as being between a man and a woman who want to procreate? Our society is more than big enough to accomodate different perspectives on marriage though apparently some religions and religious figures are not. It seems to me that their overreaction is much more of a threat to the institutions they say they value than gay marriage is. Take a look at some of the arguments on this issue: