Monte Neil Stewart (President, Marriage Law Foundation) deals with this argument here (p. 344 / 32nd page). To summarize his argument:

1. Marriage is society’s mechanism to regulate and ameliorate the consequences of passion (that is, children). Even in our contraceptive culture there are many unintended births. Marriage law isn’t to make all sex procreative, but only seeks to encourage that man-woman sex occur within marriage as a protection when it is procreative.
2. There is no procreation requirement of marriage because government has not felt that it was their place to ask.
3. During centuries marriage has encompassed the central facts of child-bearing and child-rearing and laws have been designed to regulate entry into and continuation of the child-centered institution. This has continued without a specific request that parents declare any intentions about children.

Regardless of the claims that no harm would be done with same-sex marriages, there would be significant harm to the understanding and nature of marriage and to the usefulness of that institution for society’s goals.