IMHO, that doesn't justify the practice, it condemns the book.
But if the arguments in Anamensis' previous reply was correct (decent women enjoy sex anyway, the idea that you are "raping" a woman while offering lifetime financial coverage and social respectability (technically) doesn't strike them as horrific) OR the followers of Islam were expected to be incredibly morally upright and not take advantage, it is anacceptable that their "contract" with these girls would force them - sooner or later - into marriage.