Vicky I wonder if you might be interested in my personal journey on this question.
When I first heard about same sex marriage I was against it too. But I was hard pressed to come up with any reasons that I could really defend, other than religious ones.
I do believe that religions have the right not to recognize these marriages . But from a civil law point of view there's just not a leg to stand on.
The funny thing is, gays and lesbians used to be villified for their so called promiscuous lifestyle. Now that they want to get married, some of the same people are criticizing that too.
I have to ask, what does it hurt if two people commit to loving and supporting each other for life. Surely society can only benefit from this.

I actually had a hard time coming round to this myself. Then I heard someone say -- I don't mind at all , if only they don't call it marriage. That struck me as so silly that it stuck in my mind and made me open up my mind rethink my position.