ANSWER: Is kissing a sin or not for Christians? The answer to this question all depends on the purpose of the act and what is going on in the hearts of those participating in it.
For example, it was clearly not wrong in the early New Testament church as believers used this gesture as a sign of greeting, friendship and mutual affection (Romans 16:16, 1Corinthians 16:20, 2Corinthians 13:12, 1Thessalonians 5:26, 1Peter 5:14).
In regard to kissing someone who is not a person's mate, especially in the context of dating or pursuing an extra-marital affair, Jesus revealed a basic principle on sexual morality during his famous Sermon on the Mount message.
Jesus told a crowd that had gathered, "You have heard that it was said to those in ancient times, 'You shall not commit adultery.' But I say to you, everyone who looks upon a woman to lust after her has already committed adultery with her in his heart" (Matthew 5:27 - 28, HBFV throughout).
Based on Christ's statement, when people (male or female) actively think about having sex with another person who they are not married to it is always sin, even when they do not actually commit the act. This principle is valid regardless of the age of those involved, as both young and old can lust (although the young normally do it more than those who are older).
A brief "peck" on the cheek while dating is unlikely to cause lust, such as those commonly exchanged between friends greeting each other in the Latin American culture. On the other hand, however, a sustained deep kiss (like when each person's tongue is touching) certainly is lustful.
There are many gradations in-between the above two extremes, and it is possible that what is sinful for one person will not be for another, depending on their thought processes.
Furthermore, sustained kissing is considered foreplay as preparation for physical intercourse. It may be the first "stop" for the "train" that ends with penetration and orgasm at the final destination, but it is still part of same general process. It can easily be a sexually charged "warm-up" that prepares for consummation even when the couple ultimately avoids going that far. Sometimes it is not so easy to put this act in a watertight compartment that is completely separate from sex.
One reason for single and dating Christian people to avoid strong romantic kissing is the need to protect their own and other person's purity for their (likely) marriage to someone else. If a dating relationship breaks up, and they marry someone else down the line, what kind of memories do they have when they show affection to their future wives and husbands?
Sex is such an intense experience both physically and emotionally. It is hard for people to erase their memories of what happens during it when they are finally with that "special someone" months or years later. As more parts of the sexual relationship are saved for marriage, they then become more special since they are not besmirched by guilty thoughts when done with their life partner.
For further study, see Joshua Harris' book "Boy Meets Girl," especially pages 141 - 166, where he reasons carefully about why he rejected kissing his future wife before they got married. In the final analysis, we should avoid situations and behaviors that can tempt us to sin and do things that are only permissible in marriage.