I think there is an obvious difference between a presumably fertile union that becomes infertile by disease and one that is contraceptive by design. But let us take your argument head on. Let us take the case where one fertile couple chooses to be closed to children. The best answer I can give you isn’t entirely pure, but it would be this. They are getting marriage benefits they don’t deserve.
If I knew beforehand they would have no intention of having children, and were still demanding a right to marry, I would be voting not to grant them marriage rights.

Yes I realize that a very small minority of gay couples have children from heterosexual unions. I wish them all the best and am open to supporting them in their awesome responsiblities without changing the institution of marriage.

I hate to have to revert to such an infantile argument, but would you advocate not recognizing relationships of infertile straight couples? You can string together a sentence pretty well, I don’t wish to patronize.

Let’s take two “committed contraceptive relationships” – one gay, one straight. You say that these relationships are equal. The straight one can get married, the gay one can’t. Umm.

Also, gay people have children.

It’s not just “love and commitment”.

I suppose one could make a case that in a committed contraceptive relationship (such as a gay relationship), both partners experience some postive benefits that are of value to society. For example, I believe it has been fairly well documented that partners in long term relationships have decreased morbidity and this saves society money so we should give some sort of measured benefit. I believe there are also few minor other ones that sociologists are recently promoting, but taken in the aggregate, these benefits to society hardly begin to approach the benefits that committed heterosexuals in marriage bestow upon society. (cross sex socialization, the next generation, a biological heritage… you know all that outdated stuff like that:)

The only case gay marriage supporters seem to be making these days, if they are at all willing to come out of their “marriage equality” rainbow fortress, is that based on their love and committment they deserve all the priveleges of marriage. Don’t take this personally, but they just don’t deserve it. Not because they are gay, or because their personal relationships are inferior to mine (they are not), but because they don’t do much of anything for society in exchange for the benefits they are seeking.