The unmarried man is busy with the Lords affairs, concerned with pleasing the Lord; but the married man is busy with this worlds demands and occupied with pleasing his wife. This means he is divided.
(1 Cor 7:32-33)
Good post thanks for the Scripture reference.
I think Stingray put it best in laymans terms too although of course the Scripture passage you provided puts the Churchs reasoning most compactly.
Theres nothing wrong with being married and a priest per se, its just that the Church recognizes just what St. Paul did, that an unmarried man has fewer things to concern himself with so his flock can be his primary concern more easily.
Im pretty ambivalent, as a Catholic, to this whole issue myself. I can see both sides of the issue and since its not a dogmatic issue it could go either way. But we do have many centuries of tradition that have produced many holy priests in this fashion, so I lean more to keeping the status quo.
OTOH, I really cant see any harm, except for as stated above, in allowing a man who is already married become a priest. We already let such become deacons.