Maybe because many people (actually, the vast majority of people- gays included) do want children? And think it would be nice, if they're in a long-term committed relationship, to have the protection and benefits that come from marriage?
Not all gay comic characters have been portrayed as wanting to be married or wanting children. Neither Renee or Kate, as far as I know, have expressed a desire for either. Neither have Billy or Teddy over at Marvel. There are reasons for this- Kate and Renee really don't have lifestyles seemingly conducive to raising kids and Billy and Teddy are too young to worry about that stuff. However, the couple here do have a lifestyle (almost, if they weren't so busy) where they could raise children and are at the stage in their life when people generally start seriously thinking about settling down and doing so.