It’s a matchlocks between the rambam in the beginning of hitchos eishus and the rosh in kesubos 1:12.
The rambam holds it’s a mitzvah and the rosh holds it isn’t. Mistama
That is for a man
It could be that even the rambam holds that it’s hechsher mitzvah for pru urvu because the rambam hold lishitaso that one can’t have children with a concubine. So marriage is the only way to do pru urvu