Mike Hayes, one of the editors at Bustedhalo® opined in response to this question, “I think kissing WITHOUT passion might be a sin.”
Mortal sin is “deadly sin,” sin that separates us from the community and God on a deep and real level. To persist in mortal sin, knowingly and will full intention of the will, cuts us off from the life of grace, the power of God to save us and bring us to eternal life.
It is difficult to argue that passionate kissing by itself could rise to the level of mortal sin. Now if you’re kissing another person’s spouse or your close relative, consequences may be more complicated.