No, kissing on the mouth is not a sin if it does not lead to other acts that are wrong. Each person within the relationship must know and establish their limits. A kiss given without ulterior motives in dating is not a sin.
Can kissing on the mouth lead to sex?
It depends on the hearts of people who kiss. If your intention is just to show innocent affection, it probably won’t lead to anything, but it is good not to overdo it. Prolonged physical contact ignites the desire to have sex. When this happens, it is difficult to control, because it is very strong.
The kiss given in the heat of passion will naturally lead to sex. This is because it increases a desire that is already there. The kiss is only a small part. The real cause is in the intentions of the kisser ( Ecclesiastes 10: 2 ).
Can a believer kiss on the mouth during courtship?
Yes, during courtship you can kiss on the mouth, if it does not provoke lustful desires ( Matthew 5:28 ). It is very important to set limits on dating but these limits vary from couple to couple. We all have weaknesses. Admitting that some forms of dating intimacy bothers you is a good thing. A relationship needs honesty to work.
Read here: what does God say about dating?
Those who have difficulty in controlling themselves need to establish stricter limits and perhaps avoid too much kissing, so as not to fall into sin. You must also think seriously about marriage ( 1 Corinthians 7: 9 ).