From the Catholic Encyclopedia
As I understand it, religious priests usually take vows of chastity and celibacy while secular priests only make a promise of celibacy. A vow of chastity would really be redundant, wouldn’t if, since all Christians are obligated to be chaste according to their state of life? Simple translation - no sex outside of marriage.
So, technically, the ex-seminarian is correct that a priest having unmarried sex is not breaking his vow of celibacy. He is wrong, however, in saying that this is permitted and/or not a sin. It is a most serious sin against chastity and against the 6th commandment. The promise of abstinance is implicit in the vow of celibacy since the only valid use of the marriage act is within marriage.
IMHO, what the ex-seminarian advocates is worse than breaking a vow since a vow can be annulled or dispensed but chastity is a life-long obligation.