I agree that being a mature adult should mean not resorting to crude content. There are many more ways for me to say that I loathe someone I have to deal with at work than calling them every four-letter word in the book, and I pride myself on being able to do that. (Why call someone a f**khead when you can say that they have the attention span of a two-month-old ferret on crack cocaine with a Pixy Stix chaser, and the native intelligence of a wet sock in a vacuum cleaner? Not that calling them that is mature either, but it's certainly more creative.)
But I don't think that acting and speaking as a responsible mature adult automatically means that everything you say and do is safe and/or appropriate for children to view. Granted, most of the things someone will say when they are acting in a mature and responsible fashion is going to be safe. But for that small segment of content that is not safe, the mature action is to place it out of the way of those who should not get into it, and to label it clearly so that no one is surprised by the contents. It's obviously a flawed analogy, but think of medicines, cleaning supplies, and other chemicals that should not fall into the hands of children.
And it isn't all about sex. I wouldn't be comfortable exposing children to the sort of soul-destroying desperate words that come out when someone is battling depression, especially when they're losing the battle. I think they should be spared seeing graphic descriptions (or photos) of war atrocities, invasive medical procedures, or garden-variety brutality and rape. In a perfect world, these things wouldn't happen, but it's not a perfect world, and adults do talk about these things, even in real life in public.
And on to the sex -- I think it's possible to talk about sex, and have an explicit and erotic discussion or description of sex, and still treat the subject in a mature fashion. Granted, I wouldn't yell it from the street corner, but I don't have to use crude terminology to describe it explicitly. But no matter how transcendent the sex -- and I've seen some of the stuff the original poster has created, and it's gorgeous, loving literary art -- it's still not something that I would feel comfortable telling a small child it was okay to read at that point in their lives.
And there's public and then there's public. I wouldn't start a discussion that I didn't feel was appropriate for all ages in an all-ages forum such as this community or an all-ages chatroom, or shouting it from a street corner. Would I start a discussion with some friends when we were gathered in a secluded corner of a restaurant, or in their personal journal? Definitely. It is technically a public space in that anyone could come in, but