There was a great comment in another post from someone: "some princes love princes and some princesses love princesses"
At about the age of 7, my daughter whom had seen a news program showing the annual gay parade day, asked me what is a gay person.
I simply said to her: It means who you love.. some men love men.. some men love women.. some women love women.. and some women love men. Men who love men and women who love women are called gay. That was the end of the discussion.
There was no shock or horror from her over this, and years later now at 16, she still has no emotional issues over discovering there are differences in how people love and whom they love. In fact I would say she is a kinder and more rounded young person due to the fact that she wasnt exposed to a judgement answer. She accepts people for who they are, not concerned over their gender or sexual orientation.
If I had reacted to her question with shock, or had made a judgement comment on it, she would have had more issues over a persons sexual identity than she did. She would have reacted in shock and horror whenever seeing anyone who was obviously in a gay relationship. Since I did not do this, the question was answered, and life carried on.
In my opinion when a child asks about human sexuality, it is important to discuss this openly and non judgingly in an age appropriate fashion. Just by stating a simple fact.. as I did.. would steer away any future confusion.. the child simply accepts it as how it is.
In my opinion if I had a family member whom was in a open alternate relationship, I would just explain to my child in a non judgemental way that this is just another way of loving. There is no need to go into further detail, one would not need to provide any further details on sexuality that is beyond the childs age level of understanding. Showing acceptance does not cause fear or confusion in a child, it is the reaction made by the adults around them, the judgements felt even if not spoken that can cause damage and confusion to the child.
So, although I would not send my 6 year old off with anyone for a 2 week vacation.. I would find it important that my child be educated and informed that his or her uncle is happily in love and just leave it at that. Simple explanations work quite well.