Yes.
If what I’ve read about Catholic teachign is anything to go by. I’ve read that marriage is between ONE man and ONE woman, ergo - any more women involved would make that marriage a sinful union.
Secondly its an offence to the dignity of the other women involved as the man can’t properly fulfil all the needs, emotion, physical, spiritual et cetera of his many wives. Not to mention, it would cause jealously and competition amongst his wives. Not a good foundation for a marriage.
I’m sure I saw this written in the catechism but I borrowed one from my mum and returned it a few montsh ago so I can’t go check now for you.
But end of the day - its a sin.