There is no theological problem with priests being married, but many centuries of experience showed the church that unmarried men make better priests because they can devote themselves 100% to their calling without family obligations to compete for their time and energy and emotional resources. It is an ADMINISTRATIVE rule, and there are occasionally exceptions made to it.
Your question is sort of like asking "where does it say in the Bible that ministers need to have a theological or divinity degree"? Many Christian churches require some sort of academic certification, but that is also an administrative rule which admits exceptions, not a theological requirement.
. Sorry, but I know many married men who are Pastors of churches, and they have no problem taking care of church business and their families at the same time.
Besides, if they are too busy, that's what Deacons are for, they are to be servants of the church, too, they have responsibilites to help the local church.