Aquinas condemned every form of kissing, except where it has a ceremonial or societal importance- he did so in that it is seeking sexual enjoyment apart from an opportunity to make children and with this create stronger bonds.
French kissing is foreplay.
Now, we know his reason for excluding ceremony is that kissing itself is neutral, but is often an acting of lust on one side or the other, if the pleasure is not sought- but rather a sign of love- then kissing is no sin at all- but if either side is acting out of lust- then it is sin- so maybe ask her why she kisses you, it must be as a sign, or such or it is a mortal sin, one you share in her sin, that you allow. Otherwise it is foreplay- which is meant for marriage (Humane Vitae), yet you should be very careful here.
www.chastity.com (Catholic Answers chastity site) warns that is is a place of danger and we know it can influence how easy it is to do some other things, or too early physical contact can affect a relationship. Generally- if another is prepared to forfeit this, then it is sign of real love, and not simply lust. Even non-catholics are under every rule we are, except ceremonial laws perhaps, which falls under their already rejection, which Augustine calls the blasphemy of the Holy Ghost (unless unintentional). Of course Aquinas, also allows for deliberate sin also to fall into this- sinning for the pleasure of sin.
The point is, I think for the relationship- wait til marriage.
And in most cases- it is
“I answer that, A thing is said to be a mortal works. sin in two ways. First, by reason of its species, and in this way a kiss, caress, or touch does not, of its very nature, imply a mortal sin, for it is possible to do such things without lustful pleasure, either as being the custom of one’s country, or on account of some obligation or reasonable cause. Secondly, a thing is said to be a mortal sin by reason of its cause: thus he who gives an alms, in order to lead someone into heresy, sins mortally on account of his corrupt intention. Now it has been stated above (I-II, 74, 8), that it is a mortal sin not only to consent to the act, but also to the delectation of a mortal sin. Wherefore since fornication is a mortal sin, and much more so the other kinds of lust, it follows that in such like sins not only consent to the act but also consent to the pleasure is a mortal sin. Consequently, when these kisses and caresses are done for this delectation, it follows that they are mortal sins, and only in this way are they said to be lustful. Therefore in so far as they are lustful, they are mortal sins.”
Imprimatur, Summa Theologica; Saint Thomas Aquinas (the basic Christian basis)
newadvent.org/summa/3154.htm#article4
So, I think we mostly believe we can handle things, but are putting our partner at risk also, and the sin is theres whether they are Catholic or not, so ours is their share in it- that said- I strongly advise to stop, and avoid this- the only speech of the church on this matter condemns it, and this is in line with the CCC-
Lust is all pursuit of sexual pleasure- apart from both the unitive, and the pro-creative properties. This is the definition of lust, therefore if this kissing is in any way a pursuit of sexual pleasure- which it is, then it is lust, and lust is the form of mortal sin, if not the actuality.
