You can gain a wife by courting her at a rival's home, who has a daughter. Visit his home and you will see an option on the drop down menu.
You can also visit your Daiymo at his home when the message that a matchmaker is offering a wife, appears.
The higher the wife's honor, the better. Sometimes you will have to rescue her from bandits first in order to win her.
It is crucial to marry as soon as possible so you have heirs. Keep checking your rivals bios to see if they have daughters of the appropriate age to court. Younger fertile wives will provide more children.
every point made here is why this game is romance done right. first and last of its kind