And one other thought on this topic. Regarding a man's sexual needs a wife is supposed to not deny him those. Abu Hurairah (May Allah be pleased with him) reported: Messenger of Allah (PBUH) said, "When a man calls his wife to his bed, and she does not respond and he (the husband) spends the night angry with her, the angels curse her until morning". [Al-Bukhari and Muslim]. Abu `Ali Talq bin `Ali (May Allah be pleased with him) reported: Messenger of Allah (PBUH) said, "When a man calls his wife to satisfy his desire, she must go to him even if she is occupied with the oven". [At-Tirmidhi and An-Nasa'i]. So unless the wife has health issues or something similar I don't see why having another wife for sexual needs is really necessary. It's his right to have relations with his wife and her duty to give that to him.