In all honesty, I believe 8 yrs old is a little on the older side to know the facts. Maybe your son is ready to know as many children that age are but you are not ready?
Rest assured, it is talked about at school among the boys, etc.
Yes, I would be upset if another parent discussed something that I would have preferred to do but........is it possible that this parent talked to them because they asked him about a fact? OR... Maybe this parent heard the boys discussing what I've heard kids discuss at school and he set them straight with the truth. When I hear this topic being discussed when I am teaching, I refer the subject to the guidance counselor so she can let parents know there needs to be a discussion at home or if she needs to have the discussion she can with parental permission.
I think you should be honest and make sure this child understands that sex is not dirty and nasty. If you can't convey that then get someone to talk to him who can because you do not want him growing up with the idea that sex, bodies, nudity, is nasty and dirty. That would be setting him up for issues WAY on down the line sexually.
Have his dad talk to him. Maybe he is embarrassed to talk to you.. some children are more comfortable with this type of talk with the other parent. Do whatever you can to help him understand that this is normal and a good part of life.
We kept our lines of communication way open and still do because I would much prefer my daughter to hear the truth from me than think what she hears from friends is the truth.
Best wishes... they grow up very fast and I agree that these topics should be discussed at home and not with other parents. Each family has their own balance of what they need to do and yes, the pther parent overstepped a boundary.
Move on and make sure you communicate with your child. It is a good sign that your son came to you....to talk about this. That is a positive!!