This might be better in Catholicism 101-I am not sure, but...

Is french kissing grave matter, if the guy and girl are not married?

I heard from a priest that a certain pope (can't remember who) said it was...Perhaps I misunderstood and it would be an occasion of sin? I don't know.

FWIW, Jone says it is usually a mortal sin.

That doesn't even remotely sound like something a Pope would be opining on.

I don't have much time to read through this link, but it seems to say that Pope Alexander VII said a kiss is not “merely a venial sin when performed for the sake of the carnal and sensible delight"

http://taylormarshall.com/2010/11/is-fr ... -pope.html

Thomas don't speak about french-kisses, but more general: http://www.newadvent.org/summa/3154.htm#article4

Häring treats the question in part III of his work on moral theology.

The very short answer is that it depends on the persons on how much of a danger it exists that it will lead further. Also we shouldn't try to just barely avoid mortal sins, but any sin.

"Deep kissing" (french kissing, soul kissing, etc.) is something that causes the female body to go through certain biological processes that prepare it's genitals for intercourse. The tongue and lips are very sensitive erogenous zones. These sorts of kissing are classified as "foreplay".

Engaging in foreplay outside of marriage is sinful. The degree of that sin is between that person and their confessor.