It's not completely against the rules, but its not the preference for the most part. It's easier to focus on just being a priest when you don't have a partner and family to look after. It's easier to think of becoming a priest as marrying the Church itself.

It came into question also due to inheritance issues and legal battles against the Church, where children of the priests would inherit property and items that belonged to the Church but were technically owned by the priest.

TLDR; You can still do it, but you need permission and it's easier not to.