Many circumstances can effect ones ability to give full and informed consent. In marriage that Church assumes that the marriage is valid. For Catholics, there is law that the marrigae vows should be in the church. For other Christians there is no should law, so any intention and exchange of vows is assumed valid. If intenct or understanding was lacking at the time of the vows then the marriage would not be valid.
Many circumstances can effect ones ability to give full and informed consent.