Prevalent does not make it natural, and many cultures have found ways to minimize exploitation, so there must be some agreement that it's not okay. Clearly, it's not the best part of human nature.  In my mind it's on a continuum from beneficial co-operation to cannibalism - quite a bit closer to the latter than the former - and I do believe that I'm not alone in my feelings of horrified disgust and outrage at the perpetrators.  
What to do though?
When it comes to sweatshop labour, I can inform myself and avoid products that will exploit others.  That gives me the sense that I am contributing to a much lesser extent to the suffering.  It isn't nearly enough, but it's all I know how to do.  There just isn't any way I can cut down on my consumption of sex and house slaves, since there wasn't any consumption to begin with, and I'm one of those people who is apologetic about telling people what to do, even when I'm paying them.  Slavery intrinsic to life?  No.  Humans function just fine without it.  I just don't get the slave master mind-set, and frankly - it frightens and angers me. 
While there is no sex tourism in the US, I can't be sure there isn't a woman locked up somewhere in my neighbourhood.  The fact that it's officially illegal here, makes it less prevalent, and more likely that a person can escape and ask for help.  To accept slavery, one can never clearly look at what it truly entails, can one?  It's easy to look away when it's people of another race and culture, a half a world away, but then, occasionally a person surfaces in Europe or the US who has been held in a basement or shed, and this has become their world.  I just took a moment to imagine what that might look like - if a person came up to me, and told me they just got away from someone who kept them locked up.  What would I do?  At least the person's captor cannot come forth to claim ownership, but in another country they might be able to.