I don't think these are against Islamic law, since men can have multiple wives. I'll have to ask the neighborhood mufti.
The word harem stems from the word 'haram', meaning forbidden or sacrosanct. In muslim cultures the portion of the house reserved only for women and forbidden from men were haram.
Over time westerners assosiated the word haram with the mysterious household women, thus creating the word harem, meaning a collective of women under the domain of one male patron.