"I have no problem...."
But you are not the only person on earth.
You did not consider the feelings of the other people involved, both parents and kids. While I understand your safety concerns, the thing to do would have been to crack the door, call your son over to you, give him the directions "you kids need to stop, or else we'll need to send an adult (male adult, dad or staff) in" (warning).
You understand *why* there is a separate locker room for boys, right? So, why violate the privacy that is meant to both imply and provide? I can also tell you from years upon years of working with children that guidelines (every guideline from insurance to Boy Scouts to volunteering at a school) insist that adults never be left alone with a young person who is not their own son. I can *never* go out of sight of another adult while dealing with bathroom shenanigans. So, while it was your son you spoke to, you embarrassed and violated the privacy of his friends and likely, their parents, and put yourself in a potentially compromising position. As a mom, if you'd had a problem with MY kid, I would have expected you to come to me to deal with, most esp in that situation. And you totally missed the boat--- SHE had a problem with YOU seeing *her* kid naked. So don't make it about her being hung up. She wasn't wrong, you were. We often have reasons to see much younger children naked if we are caring for them, (toileting, accidents) but even then, appropriate practice means we minimize that contact whenever possible and ask children to do the majority of that care (undressing/dressing oneself, etc.) Seven year olds do not have that sort of level of need for care unless there is some other impact (read: disability) involved.
You are not the family doctor, you are not the parent of the other children, and most of us teach our kids that these are the only people (besides peers) who should be seeing them naked at this time in their lives. For those of us who do our due diligence in teaching our children boundaries, what you did was confusing and overstepped those boundaries in a big way.
I want to add, like Suz, I don't see a big deal about the human body, but you have to remember those are other people's children's bodies, and that's where what we feel is 'okay' has to, in practice, line up with what most of our society teaches our children. It wasn't just at home, where the kids were streaking to be funny. It was in a public place with other people's children. *That* is where I draw the line.