Maybe he was just indicating that she may be married, and that looking at a married woman lustfully is the same thing as coveting, which is a sin?
Chapter and verse
All lusts are a sin. They distract us from God. Even the lust of a husband and wife can be sinful if it is not used for its purpose of bringing us in to God's joyful creation. In all that we do, God must stand at the apex.
+ffs, IN BRIEF
1659 St. Paul said: "Husbands, love your wives, as Christ loved the Church. . . . This is a great mystery, and I mean in reference to Christ and the Church" (Eph 5:25, 32).
1660 The marriage covenant, by which a man and a woman form with each other an intimate communion of life and love, has been founded and endowed with its own special laws by the Creator. By its very nature it is ordered to the good of the couple, as well as to the generation and education of children. Christ the Lord raised marriage between the baptized to the dignity of a sacrament (cf. CIC, can. 1055 # 1; cf. GS 48 # 1).
1661 The sacrament of Matrimony signifies the union of Christ and the Church. It gives spouses the grace to love each other with the love with which Christ has loved his Church; the grace of the sacrament thus perfects the human love of the spouses, strengthens their indissoluble unity, and sanctifies them on the way to eternal life (cf. Council of Trent: DS 1799).
1662 Marriage is based on the consent of the contracting parties, that is, on their will to give themselves, each to the other, mutually and definitively, in order to live a covenant of faithful and fruitful love.
1663 Since marriage establishes the couple in a public state of life in the Church, it is fitting that its celebration be public, in the framework of a liturgical celebration, before the priest (or a witness authorized by the Church), the witnesses, and the assembly of the faithful.
1664 Unity, indissolubility, and openness to fertility are essential to marriage. Polygamy is incompatible with the unity of marriage; divorce separates what God has joined together; the refusal of fertility turns married life away from its "supreme gift," the child (GS 50 # 1).
1665 The remarriage of persons divorced from a living, lawful spouse contravenes the plan and law of God as taught by Christ. They are not separated from the Church, but they cannot receive Eucharistic communion. They will lead Christian lives especially by educating their children in the faith.
1666 The Christian home is the place where children receive the first proclamation of the faith. For this reason the family home is rightly called "the domestic church," a community of grace and prayer, a school of human virtues and of Christian charity.