This great synod absolutely forbids a bishop, presbyter, deacon or any of the clergy to keep a woman who has been brought in to live with him, with the exception of course of his mother or sister or aunt, or of any person who is above suspicion.

This is what i was referring to, but it may have been a later council? It’s canon from what I’ve read, but not specifically about marriage, but “keeping a woman”.

https://www.papalencyclicals.net/councils/ecum01.htm