In Adhania, the prevalent culture was that children of mistresses could almost never replace that of main wives.
Even if the main wife was unable to bear children, it was more socially acceptable for her to adopt a child of one of her husband's mistresses than for a husband to let one of his concubines take his wife's place.
Hence although the four were not thrilled over their husband's womanizing ways, the hope was that by letting Alexander fraternize with only the maids, they could at least contain his desires, preventing him from looking at the many much more powerful noble women around him.
The four far more preferred for Alexander to favor one of these maids rather than a noble lady because he could make the former only his concubines and without any powerful mother family, any children born would be too low to be of anything more than a steward or butler in life.