Kissing becomes a sin when you know so well in your heart, mind, body and soul that your thoughts regarding the person you practicing it with aren't so pure. Why do I say this? Now let's have a glance into the bible where we get to read the instructions of God as telling man not to harbor in their hearts lustful thoughts. The book of Matthew 5:28 outlines Gods instructions to man to abstain from any thought that leads to the committing of sin. 'But I say unto you, that whosoever looks at a woman to lust after her hath committed adultery with her already in his heart.' You don't necessarily have to get that physical with someone before you get to commit the sin of fornication with them. The more your thoughts of negativity outdo your whole being; you end up with no choice but to succumb to the desires of your flesh. To have that passionate attraction towards another in an impure manner is a definite way of committing to worldly pleasures which can be equated to sin. Kissing is not an exception as clearly seen here and its practice when leading one into stepping outside that boundary of being purely innocent, and entering that phase of becoming negative, filthy and impure, then automatically becomes a sin.
The bible says that every sin a man commits is outside his body, but he that commits fornication sins against his own body. 1 Corinthians 6:18. Your whole emotion centered on kissing someone becomes sinful when you manage to cross the line of taking things too far and deep. Be not consumed by thoughts that defy your being and whole outlook as a Christian. Lust is clearly a sin and the bible never failed to hammer on this point of Christian growth and development. So the question isn't really concentrated on one just kissing, but it is about what you feel in your heart and your thoughts directed towards that person with which you doing it with. You need to go deep within yourself and really ascertain whether you are doing the right thing or not being the Christian you claim to be. Ask yourself these simple questions to help you in identifying where your fault lies when you practice this act. Do you have pure thoughts when kissing someone? Do you feel the urge and need to go further with that person aside the kissing aspect? If you answer positively to these questions, then by all means know that you are committing a grave sin when and if you kiss with a lustful mindset backing it. While not all kisses in relationships are sinful, there's the need to exercise caution in any relationship you find yourself in just so you don't go against the word of God and end up defiling your being. Absolute self-control is needed here if you ask me especially if you know you are the kind who can't seem to restrain from going further after initiating a kiss. Kissing, should not lead you to the point of committing a sin. Temptation arises when you allow yourself yield to lustful desires of the body and flesh.