If your wife cannot have children, the only way you can get another wife is to divorce, or assassinate, the current one. You get a divorce by appealing to the Pope, although a bribe or threat may also help. There is a menu option for this, although a personal message to the Pope may help speed his approval.

If you can't get a divorce, you can try to assassinate your wife. If caught, you're in big trouble. Note that successful divorce requires that you return the dowry, which could be expensive. About 20% of wives are infertile.