Your coworker’s marital status is actually insignificant – the fact that you describe your behavior as “leering” suggests that it has gone beyond appropriate admiration and become an expression of lust. How can you tell the difference? A good clue would be that you would be embarrassed if your coworker found out what you are doing. In Catholic language, we might say you are showing a lack of chastity, the virtue which seeks to integrate sexuality within the person. You have introduced an inappropriate sexual element to your relationship with your coworker, even if it is unbeknownst to him/her. Rather than encouraging your own healthy integration of your sexuality, your behavior drives a wedge between your sexual feelings (which you express by leering at your coworker) and who you really are (presumably, a friendly colleague who maintains appropriate boundaries with this person.) Related to the virtue of chastity is modesty, which protects the mystery of persons and guides one’s behavior towards others, in keeping with their human dignity and one’s own. Read more in the Catechism of the Catholic Church, paragraphs 2337-2351 and 2520-2524.
It can be difficult to change this kind of behavior on your own, especially if you are struggling with chastity and modesty in other areas of your life. Recognizing that what you are doing is wrong and seeking to change is an essential first step. Pray that God will give you the grace to integrate your sexuality in your life in a healthy way. And don’t hesitate to consult a spiritual advisor or therapist for additional help.