I'm not the poster you're quoting, but yes, we ask the parents of any child before our children go over to their home to play whether there are guns in the house. If the answer is yes (and it has only been "yes" twice), we ask for the child to come to our home or to meet somewhere else to get together. It has never been an issue, and we assume the parents are not lying to us when we ask. We explain that it has nothing to do with our trust of the parents, but that it's a personal value, and both gun owners were completely understanding and our daughter is friends with both girls. They merely play at our home or at a third-party location.
Yes, my wife and I ask. We don't think people who own guns are bad people, but guns are not compatible with our value system, just as some parents eschew having their children visit homes where there is alcohol (using that as an example of something else that is legal and usually enjoyed responsibly but to which some people have an objection on principle). We don't know any police officers or members of the military and our children have not made friends with any children of police officers or of people in the military as of yet, so that is an interesting question that we will likely address if and when the time comes. My candid reaction, however, is that no, we would still not allow our children to be in their home at least initially but if we felt comfortable with the parents, we might reassess in the future.
For my wife and me, it is a values-based choice. We simply do not like guns and they do not fit within our life or our family's values. A pool, while certainly dangerous, is not inherently designed to kill. Also, to be fair, we do ask whether the parents take appropriate precautions if they are going to swim at a friend's pool, but we will generally suggest going to our country club to swim, instead, as we know the lifeguards well and are more comfortable with them. Since nearly all of DD's friends belong as well, that usually works for all involved.
Again, it's just a values thing for me and my family. I don't think gun-owners are bad people and I know that most use their guns responsibly. Honestly, yes I do wish there were fewer guns and more restrictions on them, but that is not the law of the land and I accept that. As a pacifist, however, I find them incompatible with my values so rather than insist they be removed from the world, I choose to remove them from my life.
And yes, we speak to our kids about guns, that they are not bad but that they are very dangerous, and to never play with one if they find it. When they are older, perhaps even as teenagers if they show responsibility and competency, they will make their own value decisions about guns and may choose to use them. But for the time being, I do not want my children playing in a home with a gun.