What constitutes going too far...for me...are snuff and human chattel stories and extreme of the extreme mutilation and torture stories (think medieval "witch" torture stories). Those I cant stomach at all.
With everything else it depends on how well it is written. And not all serve the same purpose. Some I read from purely artistic point of view...some are very life like and play out what I would want done to me...some are completely unrealistic, but they still turn me on...some, I don't mind, but they do absolutely nothing for me...some make me go "yuck" or "ouch", but whatever turns you on. Point is...as long as you draw the distinct line between fantasy and reality you are good, all is good...even shrinks always say that to fantasize is a healthy outlet...90% of what I read I wouldn't want, or am too much of a wuss to ever agree, to have it done in reality (some are even hard limits).
Rape stories...for me personally fall into "I don't mind, but they do absolutely nothing for me" category;...being forced...being over powered...yes, oh yes, but rape stories...I do get the appeal intellectually, but they leave me bland. That however is just me, its one of the most common fantasies (or at least thats what they say in studies), and a lot of women get off on it.
I don't like depressing, hopeless realism when it comes to literature in general. I do appreciate it from intellectual point of view, but its not something I enjoy reading. If I want depressing, I watch the news...there (in reality) I can deal with it, but not in fiction. A lot of it has to do with the fact that the good chunk of my formative years was spent in country torn by war, in situations like that you are focused on survival, hopelessness is a luxury you cant afford when you have people depending on you. I do have my dark mood days, but giving up (in spirit) is a foreign concept to me.