Speaking as a parent who has left his children to be babysat at houses with both guns and pools, the pools are a far bigger concern. When both are treated as the dangers they can be, neither is a threat. It's much easier to forget about a pool than a gun, though, and a gun is much less likely to be accessible to a child than a pool. Kids who can't swim can fall into a pool and drown before you realize that anything's wrong, and playing around pools is normal kid behavior, while playing with Actual Real Firearms is not.
I wouldn't leave my kids at any place that I felt was going to be a tangible danger to them - be that a house with a pool and a back door that doesn't have a child lock or pool gate, or a house of an irresponsible gun owner, or a house with exposed electrical wiring, or anything else that I feel poses a material threat to my child's life.
Many parents would rather that their children not visit the home of a friend that had guns in it. Would as many parents be apprehensive about sending their children to a home with a pool? I suspect not.
There may be legitimate reasons that they are treated differently in a way that does not immediately make sense when you examine the risks they pose, but I don't think that choice or lack of it is that reason. I think that you are looking for something more subtle.