Point of clarification...
I would like to add my own personal spin to this (since I guess I inspired it in the first place)...
I feel there is a difference between a book that is ABOUT "mature content" (e.g., BoVD) and a book that CONTAINS "mature content."
Of course, your definition of "mature content" may vary.
For example, a fairly recent issue of Dungeon magazine contained an adventure where the two primary servants of evil were brother and sister -- and the sister was carrying the brother's child. To me, a story involving incest is a story ABOUT "mature content." However, in my opinion the article/adventure did not CONTAIN mature content (subtle difference).
Conversely, the foreword of Liber Bestarius (which I found somewhat objectionable) was not ABOUT "mature content" (unless one feels that talking about playing RPGs is talking about mature content) yet it did CONTAIN "mature content" in the form of certain four-letter words.
Sometimes the two are mixed (e.g., something that contains mature content and is about mature content) but I think there is a very definite distinction. Where do we find the "mature content?" In the subject matter itself ("about") or in the text and/or illustrations of a work ("contains")? I believe there are certain words and/or illustrations that are objectionable, separate entirely from all content.
My vote for the above survey is as follows:
I neither object to nor have a problem with buying works that are ABOUT mature content. In fact, they can be the most rewarding, as they let us explore the darker side of human nature (things are not always bunnies and puppies).
I do object to and try to avoid purchasing works that CONTAIN mature content. I feel that if you can't express your ideas clearly without stooping to "mature content," then I'm really not interested in what you have to say.
I hope I have made the difference clear. You, of course, may have different views of exactly what qualifies as "mature content" and that is of course your right - everyone will have a different "line in the sand" I think. But IMO, the general principle holds true - it is possible to discuss sexual functions tastefully (e.g., medical textbook) or crassly (e.g., jokes at a bar). It's the same material, just a different presentation. Anything I "invite into my bookshelf" had darn well clean itself up before I invite it in.
--The Sigil