My mother told me years ago that there is only one reason for marriage.... having kids. Obviously two gays can't have kids together without involving a third party (a surrogate mother). Either they still are stuck on that reason or they are being anti-gay. My feelings is on the latter reason.
Adoption.
Not to mention that having kids might have been her reason for getting married, but it's not everyone's reason for getting married. There are people that marry that don't have kids.