3 Therefore (thus) says the Apostle, the teacher of the Gentiles, the vessel of election, “But I say unto the unmarried and the widows, that it is good for them, if they shall have so continued, even as I also.” These words are to be so understood, as that we think not that widows ought not to be called unmarried, in that they seem to have made trial of marriage: for by the name of unmarried women he means those, who are not now bound by marriage, whether they have been, or whether they have not been so.
And this in another place he opens, where he says, “Divided is a woman unmarried and a virgin.” Assuredly when he adds a virgin also, what would he have understood by an unmarried woman, but a widow? Whence also, in what follows, under the one term “unmarried” he embraces both professions, saying, “She who is unmarried is careful of the things of the Lord, how to please the Lord: but she who is married is careful of the things of the world, how to please her husband.” Certainly by the unmarried he would have understood, not only her who has never married, but her also, who, being by widowhood set free from the bond of marriage, has ceased to be married; for on this account also he calls not married, save her, who has an husband; not her also, who has had, and has not.
Wherefore every widow is unmarried; but, because not every unmarried woman is a widow, for there are virgins also; therefore he has here set both, where he says, “But I say unto the unmarried and the widows;” as if he should say, What I say unto the unmarried, I say not unto them alone, who are virgins, but unto them also who are widows; “that it is good for them, if they shall have so continued, even as also I.”
Source: On the Good of Widowhood (New Advent)