Dear Shana:

Impure thoughts are lustful thoughts. This involves thinking about sexual relations with someone, perhaps thinking about how to seduce the person, and fantasizing having sex with that person.

As I understand it, thinking about kissing a potential dating partner, or a person one is actively dating, is not necessarily an impure thought unless that imagery includes "heavy petting".

Even a relatively benign thought of kissing someone, however, is impure if we know the person is married, or otherwise not available for a relationship (such as a priest or nun or child) is "impure".

Thoughts of kissing random people on the street or those whom we met, outside of the context of an genuine interest in asking such a person on a date, if we entertain such thoughts, would also be considered an "impure" thought.

God Bless,
Bro. Ignatius Mary