That would depend on so many variables. . . for example, some lesbian oriented families have a "dad" involved. Some have two mommies, some have one mommy. Then there's the age and maturity level of the child and whether specific "examples" were available and known to the child.
I would explain that there are lots of different family units other than what was considered the traditional mommy, daddy and children straight unit. In explaining briefly that there are single parent families, I'd also mention that there are mommy+mommy and mommy+daddy and daddy+daddy famillies and that the important thing is not how many mommies or daddies are present at home, but that whoever is there is loving, caring and interested in the child. I wouldn't go overboard. I'd just explain it in the same context of some kids are born with straight hair, some with curly, some with blue eyes, some with brown, some with dark skin and some with light, and some with a mommy and daddy and some with two daddy figures or two mommy figures. No big deal.
Whew. Okay, pecan pie has my mouth watering. Now, tell me what (if anything) you did for St. Patrick's Day...