Dear Jake:

Good question.

Merely looking at a woman is not sin. As one priest said, "Glance at a woman once is appreciating God's creature; to look a second time is lust." Though looking once can cause lust in some people.

It is a sin to lust.  If looking at a woman in a bikini or revealing dress causes you to lust after them, to think lustful thoughts, then you need to "bounce your eyes" away from them. Or, if talking to her, discipline to look at her in the eyes, or at least the face. This will take discipline and practice, but it is something you must do.

Lust is a grave sin, a violation of the 6th Commandment (do not commit adultery) as Jesus said in Matthew 5:28, "But I say to you that every one who looks at a woman lustfully has already committed adultery with her in his heart." 

So, if you are lusting, you need to confess it in the Sacrament.

God Bless,
Bro. Ignatius Mary