A more thorough explanation:

Definition: Ex causa potestatis is a term used in Roman law to describe a situation where a person is restricted from marrying someone who is under their guardianship or control due to their position of authority.

For example, a tutor or curator could not marry their female ward until their office had terminated, or unless their accounts had been passed. Similarly, a person administering a government or public office in a province, and the members of their family, were not allowed to intermarry with a person domiciled in their province, unless they had been betrothed to each other before the person had accepted the office.

These examples illustrate how ex causa potestatis was used to prevent people in positions of power from taking advantage of their authority to marry someone who was under their control or influence.