More questions

Via Blogdex, I've just found Don't Amend, a campaign to stop the proposed amendement to the US Constitution that would completely outlaw gay marriage in the US.

Now, there's been lots of writing about this issue over the last week, and there'll no doubt be much more. (Chris Brooke not only has words but pictures too while yet another strange quote of the day from Cuthbertson has prompted an, ahem, interesting debate in the comments)

But, I have to admit to being completely perplexed by one of the assertions in this debate. I suspect I'm not alone, but can anyone supply me with a clear, reasoned, explanation as to why allowing gay people to get married will weaken the already existing institution of heterosexual marriage? (Note: Any answers using 'God' within them will be judged to have failed the 'clear and reasoned' test unless the argument still looks sensible if 'God' is replaced by 'A pink elephant from Alpha Centauri called Zibbit') You see, I just can't understand how letting other people get married weakens the institution of marriage. Are there going to be thousands of couples getting divorced because of it? Are there going to be men thinking 'hold on, I was going to marry this woman I'm in love with, but now gay marriage is legal I shall go and marry a man instead'? Was the legalisation of homosexuality in the 60s accompanied by thousands of straight men and women suddenly, spontaneously changing their sexual preference?