In my opinion, the essence of "marriage" is itself a commitment, or a promise made by the 2 individuals involved, to each other. As conscious beings, why would anyone but the 2 individuals involved, be it man and woman, man and man, or woman and woman, have "ANY" say whatsoever in the existence of this commitment?
It is their lives, and as long as it doesn't overshadow or hinder the freedoms, rights of other individuals, there should be no reason to disallow same-sex marriage.
Now comes the problem of "A gay couple is not a good environment for a child to grow in".
I think this argument is not valid, and that this idea is only engendered by prejudice and misconceptions of our current society. If same-sex marriage were to be accepted, then our culture and education will slowly encompass these ideas, and prejudice about it will slowly regress and eventually disappear. The same way that the concept that the Earth revolves around the Sun, and not the contrary, was first cast out as heresy by The Church, it will take years to incorporate same-sex marriage into our culture.
It is a fact that we will always be afraid of the unknown, or in this case of someone who is different than us, but how do you say who is "right" and who is "wrong"? The easiest answer is not to do so, let someone else make the decision for us; hence, where religion comes into play.. But I will always prefer rational thinking, and logic, even if sometimes it may leave some questions unanswered..
I am not trying to bash religion here, but just saying that it is entirely a personal choice, and this choice cannot be reflected or forced onto others as a general truth or fact.
Hope I didn't steer too much off-topic, Cheers