I don't doubt it. Whether is called to serve in a married capacity or a celibate one is up to them. No one is saying that clergy cannot be married. The fact that are there are a small minority of Catholic priests speaks of this fact. However, the argument is that celibacy is a calling that has a unique role in the life of the Church. I would maintain that the celibate calling is a higher one- not by any merit of those God- but by a gift of God's grace.
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.
It is not just an issue of how busy they would be (although that is an issue.) The issue is one of devotion. One who has the ability to lovingly give everything to Christ (including the joy of marriage) has the abilility to serve in a spectatular single-minded fashion.