One of the sensitive issues confronting young Christian couples is the expression of affection through kissing. We're not talking here about a simple peck on the cheek. Jesus and his disciples kissed each other as a form of greeting which is safe if done without malice.
We're talking about deep kissing on the mouth which couples normally do, based on worldly standards. But what does God's standard say? It warns believers against succumbing to the call of the flesh (Galatians 5:16) — and kissing lets you do that, especially if involves some tongue action.
Some would argue that a lingering kiss on the mouth is not sinful considering it is far from the sexual act. But sinning is not limited to doing the act per se; it also involves entertaining unclean thoughts. Kissing commonly leads to other acts like necking and petting, which, while may not lead to pregnancy, causes the actors to sin through lust (2 Timothy 2:22).
Some girls claim they don't easily get turned on by locking lips with their boyfriends. They also stop once the "boundary" is being crossed. Even if they don't get turned on, the Bible warns against causing others to stumble (1 Corinthians 10:32).
Still, others would insist on being committed to each other to the point of being ready to marry if the safe zone is breached. But that is putting the cart before the horse. If that were the case, it would be better for the couple to just marry so they no longer have to deceive and disrespect God (1 Corinthians 7:9).
So how can couples express their affection that only they can perform? Some Christians do a quick kiss on the mouth by letting their lips touch for about a second. It's sweet and intimate but fleeting so as not to arouse unwanted urges and desires. It may not be that exciting, but guys can take comfort with the thought that they are not tainting their girl.