It is important that children be raised as Muslims, and the proper domestic situation can only be ensured if the husband is a Muslim. Muhammad's rules for war state that it must be waged only against soldiers, and not against innocent parties such as citizens and Nature. Islam restricts polygamy, in that a man is allowed to have no more than 4 wives, and only if he is able to support them. A second wife is also a much higher position than a mistress. Polygamy is also a rare exception.
Most Muslim men are monogamous.
Historically, polygamy was an act of mercy, when men would marry women whose husbands were killed in war, and other unsupported ladies. It is important for everyone to study it.