Even if it were true that very few same sex couples are interested in long term, committed relationships, why should that prevent those who ARE interested in such a relationship from getting married? Granted, some will stray and become untrue to their partners, kinda like some straight couples, some will fall out of love, kinda like some straight couples, but some will remain faithful to the end, kinda like some straight couples.
If the high risk of failure is a reason for preventing marriage, then let's say that divorcees should not be able to marry. They've already proven that they aren't a good risk for a successful marriage.

Ake, I didn't mean to imply that anyone had said that same sex marriages would increase the divorce rate among opposite sex marriages, but many people have said that allowing same sex marriages would ruin the institution of marriage itself. I said that since it became legal for same sex couples to marry in Canada it has had no negative effect on my marriage nor on any marriage that I know of, therefore how can it be ruining the institution of marriage.

It still baffles me why you or anyone else objects to same sex marriage. How does it affect you? The only way it has affected me is to get me an extra couple of wedding gigs that wouldn't have otherwise had.