I think this is the perfect opportunity to find out what's inside the minds of children. If I were your friend, I'd position the question right back to the child, "why do you think he has two dads?"Trust me, most kids have already formed in their heads a certain perception about things based on their own filters (i.e. television, parents, etc.).
I have a 13 year old son, and ever since he first began talking, which wasn't until he was 4, I have embedded within him the ability to think critically. I ask him his thoughts about things, and we discuss why some things aren't a good idea. Sure we influence our children, but with all the world is throwing at them at such young ages, we really need to get a good footing into what they already understand. You'll be surprised at the response - even from a child.