I have not read all the responses, so I'm sorry if this has been said, but the books are really not representative of what happens in the BDSM world. I have spent time in that lifestyle. And for what it's worth, many many males are submissives-to females and to other males. Many people are also "switches", which means they alternate between being dominant and submissive. For some, this is simply sexual play and for others it is a full time lifestyle. Either way, from my personal experience, there are very strict rules for safety in this community.
Abuse is never ok. NEVER. but BDSM is not abuse. The books portray Ana as unwilling and she consents to acts she is uncomfortable with. A real Dom would never continue that.
I do not think that books or movies with this kind of content should be anywhere near children. But then again, I don't think that many of the music videos and song lyrics about b*itches and ho's and drugs and violence should be so available to children. I don't think that country music, with it's penchant for getting blind drunk at every opportunity is healthy for children either. 50 shades of Grey is the least of our worries when it comes to what our society has been exposing children to for a very long time.