|I don't know any parents who let their kids' friends play in traffic or watch X-rated movies.
I don't know any parents who let their OWN kids play in traffic or watch X-rated movies, either. And I think this raises a point that I haven't really seen made in this discussion.
There is a certain societal consensus that some things are NOT acceptable for children (X-rated movies, vodka shots, cigarettes); some things are highly controversial and objectionable to many parents (violent video games, R-rated movies, trips to the riflery range, sips of beer); and some things are good clean fun though not necessarily what you'd give your kids every day (rides on the ferris wheel, PG-rated movies, sweets).
As a parent, you can mostly take it for granted that other parents will not share stuff in that first category with your child. If another parent were to take your child to an X-rated movie, you would hit the roof. You might even file charges. No WAY is that within the "well, if you didn't DISCUSS it first..." range of things you expect as a parent.
The second category is slightly more iffy. Some of the stuff in it is probably a given (you may give your own kid sips of beer, but no one else is likely to), while other stuff you may have to specify (there are a lot of parents that let their kids play whatever video game you want). At the very LEAST, though, if you specify to another parent when dropping your kid off, "Throckmorton isn't allowed to play violent video games," the other parent will probably nod understandingly.
The third category, you're going to be paddling upstream. If you have a violent objection to Disney movies, you can keep them out of your own home (with effort), but when your child goes to a friend's house, the friend's parents are unlikely to call you to make sure it's okay before popping in a video of "The Little Mermaid." The general societal consensus is that Disney movies are fine for kids. If you have a very, very sensitive child, you will for a while be able to get away with telling parents, "Muriel has terrible nightmares and you'd be amazed at what sets her off. She cried for weeks after seeing ten minutes of 'Beauty and the Beast.' So please check with me before showing her anything stronger than Teletubbies, okay?"
But first off, when it's something in Category #3, it's NOT reasonable to assume that the other parents will consult you first, any more than you'd consult them before serving a visiting child organic rice milk, or letting them play with Legos, or putting on a Beatles CD.
Sometimes we have really good reasons to ban stuff in Category #3. An allergy, or a religious prohibition, or a moral restriction like vegetarianism. But I have to admit that if another parent told me that her child was not allowed to have soda, just because, I might react a little badly. We don't actually keep soda around, so soda wouldn't be a problem, but I would wonder what else would turn out to be a horrible no-no in that household. If I bake cookies, would I be expected not to serve any to that child? Is this person going to have some violent objection to plastic toys? Etc.
We do our children a disservice when we try to control their lives too much, to micromanage their relationships too much.