In such a case you obviously do not trust your spouse, which goes back to the initial question of why did you marry them in the first place?
Marrying isn't harmonious everywhere. In some places, marrying someone is not made with both spouses' consent. In Saudi Arabia and Afghanistan, forced marriages and child marriages are common phenomena. Also, in almost every part of the world, married couples have benefits such as social security, health care and other tax benefits. I don't want to argue someone should marry another for those reasons, but I'm sure some do.