The term "family" is often used to refer to people who happen to share a household, but the real meaning goes much deeper; it refers to a collection of people, wherever residing, who are connected by blood or regarded as being so.
Any person of proper breeding will have one mother, one father, two grandmothers, and two grandfathers. Whether or not the individuals are alive, and whether roles are physically biological or merely socioligical, a person's ancestry represents a perpetual and unchangeable part of the person's identity.
In another thread, I suggested one case where I thought a homosexual person should probably be allowed to adopt: if the only living relative of an orphan happens to be gay, that should not disqualify that person from adopting his nephew/cousin/whatever. And I think I just realized part of why I would view that as acceptable: because the child would recognize his ancestry as having come through his mother and father, even if they were no longer around to raise him. That, of course, is a very different situation from what would be seen in most homosexual adoptions were the floodgates to be opened.