I'm sorry but in the west a wife wasn't a mans property, neither was they in most of the Islamic world. They was subject to their husband, but they wasn't property.
Well, I was rather talking about the fact that quite a few men did treat their wives as if they were property or not much better, certainly not all of them, but those that did most often got away with it easily. Much depends on the man's character, if he loved his wife or not, and on her willpower to resist if needed. (And they
were expected to obey their husbands, official property or not...)
Anyway, the main point was that a concubines (usually) is no more of a sex-slave than a wife is. A man often would have been in a politically arranged loveless marriage while seeking his romantic satisfaction with a concubine he couldn't legally marry. In short, a concubine where they officially exist is almost everything a wife is but with less inheritance rights for her and her children. And, usually, a lesser social status, which still would be better than that of a low ranking noble if you are the king's concubine.
The modern equivalent of a concubine is not a sex-slave forced into it, but either a mistress whom you love or your favourite high-class escort (often even with an exclusive contract.)