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 (a bribe or threat may also help). There's a menu option for this, although a personal message to the pope may help speed 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.