Interesting. I had not heard that theory. I was taught that the reason St. Joseph was chosen was because of his great love of God and his purity of heart, as demonstrated in the Litany of Loretto ~ "St. Joseph Her most chaste spouse". Its also the reason that in all the pictures, statues etc. of St. Joseph, he always holds a white lily, the symbol of purity.
Catholics are permitted to hold to either point of view (as long as they do maintain the perpetual virginity of the BVM) as the Church has not formally come down on either side. It has tended to be that the Latins adopted the "adelphoi" argument, whereas the Greeks adopted the widowhood argument, but both have been viewed as acceptable in East and West.
Re St. Joseph's chastity - if he was a widower with other children, it is still quite possible to be chaste within marriage and father children. In fact all of us who are married are called to use our marriages chastely, and the chaste use of marriage does not affect our purity.
Obviously, even if he was a widower, after he took Mary into his home, he became celibate as well as chaste.