Answer:
We don't.
Step-by-step explanation:
There aren't any laws nowadays that I know of that TELL you what family is. The gay peeps got their rights as well, so that's an additional statement to that. By nature, a male and a female get together to produce babies, which in turn is considered a family unit. Take lions and wolves for example. That's in nature though, humans are a whoooooole other bag of beans.
In the U.S., where you can LEGALLY marry a "s3xdoll" (that HAS happened) and adopt kittens and rag dolls as your children and call yourself a family, then I feel comfortable enough to say that there aren't any laws to tell us what family is. To each their own, we have freedoms here.
Of course, to get stuff like medicare and money from the government, you need HUMAN children and a HUMAN partner. That's the only "law" I can think of that might come into play. Hope this helped.