"wont someone think of the children" arguments don't belong in most debates, including this one!
I'm incensed that people keep bringing it up!
You see this is where the debate falls flat on its face ... It has to be discussed.
Marriage is all about settling down, having a family, and bringing up kids. Sure it does not always work out like that for some, but having kids is one of the primary objectives of a marriage.
I think your position has some truths in it.
Not all heterosexual couples enter into marriage with the intention of having children - in fact, a significant proportion of the heterosexual population who marry may a) have already had children, or b) do not intend to have children at all. I got married as a symbol of my love and dedication to my partner, and as a way to celebrate our relationship with our closest family and friends - not to legitamise any children we may or may not have had.
"Having kids is one of the primary objectives of a marriage" - if for the moment we consider this to be the truth (which, as I've stated I'm not sure is the case), then the ability to have children and raise a family should be equally important to gay and lesbian marriages, as it is to heterosexual marriages. This is why the current situation of civil unions isn't quite good enough.