Is a girl's kissing the mouth of the one she loves at the age of fifteen, and then marrying him later, considered a "bad deed" counted against the young man?

The act you committed is a sin. It is not permissible for a woman to touch a man who is not her mahram, whether by kissing or otherwise. The Prophet, peace and blessings be upon him, called such an act zina. In the hadith: "The zina of the hand is touching," and what is meant by touching here is grasping or striking. If this applies to touching with the hand, then the matter is more severe concerning kissing. Therefore, what you did is impermissible for you, and you are sinful for it if you are an adult. Similarly, the one with whom you did it is also sinful if he was not coerced. It is incumbent upon both of you to repent to Allah Almighty for what you committed before marriage.