This is a difficult situation. The thing you have to keep in mind, is how are you going to shelter your son from these situations, either in the neighborhood, and at school. He is in school and will be coming into contact more and more with these different life styles.
Also is it fair to alienate your son from another child his own age, simply because the parents have a life style of which you do not approve or do not adhere to.
AND you have the right to raise your child in an environment consistent with you values, which is reasonable to expect.
I can not give you specific words, because I do not know your son and how you and your son interact.
I can only tell you how I have handled it with my own children, and how I have learned through my studies and some of my clients.
1. If you show a fear of the other, it will trigger curiosity, and will not have the effect you want.
2. If you show acceptance, then it also may have the wrong effect.
so what do you do?
You explain to your child that the other child is welcome to come and visit your home, but that for now, you can not let him go over to their house, until you know them better.
If he asks why, simply let him know that they have a life style which you do not approve of.
If he asks about that, then simply explain how every family is different, and he does not live with his mommy, but with his dad, and his dad's friend.
At age six, that will likely be enough.
My own children grew up heterosexual, and they were associated with gay families in Germany during our four year stay overseas. Once I got to know the families, we had an understanding that there was no public display of affection when my sons were visiting. They respected that. Part of your solution could be to have a frank honest discussion with the same sex couple.
If they are reasonably sensative to you and your feelings, and respectful, they may agree to accomodate you.
In the end, if you do not let your child play with the other child at all, the two will find a way to play somehow...it will just happen. It turns in to a forbidden fruit kind of huristic.