I believe it is a sin to kiss before married.
On what do you base your belief that it is a sin to kiss before married? There is nothing in the Bible that even hints that it would be a sin. On the contrary, kissing is a sign of respect or friendship in most of the world, and was so when the Bible was written. Most references to kissing in the Bible are to men kissing men, to women kissing women, or to parents and children kissing. Paul tells Christians to greet each other (male or female, presumably) with a "holy kiss." (Romans 16:16; 1 Corinthians 16:20; 2 Corinthians 13:12; 1 Thessalonians 5:26)
There is one example of a man kissing a woman, and it would seem to argue against your belief. When Jacob had traveled to his uncle's lands he met Rachel, who would later become his wife. The scripture says, "And Jacob kissed Rachel, and lifted up his voice, and wept." (Genesis 29:11) This was even before he told her who he was, and seven years before they got married. There is no indication that it was a sin for him to do so.