storyline - Is BDSM becoming mainstream?

Seems that in the last decade there is more and more stories with a BDSM arc.

For instance:

• The Fifty Shades series


• The Sword of Truth series


• The Black Dagger Brotherhood series

I remember when I first read the Wizard’s First Rule and follow-ups; I was mildly shocked that it was a great success in spite of the sexual content.
(Since then I have been thinking that it may be because of the sexual content that the novels were so well received.)

So, is this a trend? Is exploring BDSM mores becoming more conventional?

Or is it just that in the aforementioned cases it is a direct result of cross-genre writing ( i.e. romance+erotica; fantasy+erotica; dark fantasy+detective+romance+erotica and other +s for the rather messy J.R. Ward stories)