The first time Sebastian kissed Kurt Hummel it was a complete accident. Personally, he blamed Finn for pushing him into Kurt as an "accident" and it caused their lips to meet.
Of course Sebastian didn't complain. The pair have been going out for three weeks now and both were too shy to make the move. Usually Sebastian would have done it but every time he tried, he would always get interrupted or someone would gain Kurt's attention.
The night that it happened was when they reluctantly agreed to join Finn and Rachel on the double date. The date was fun and all four got along and had a good time. By the end of the night, they were all laughing and that is when Finn "fell" into Sebastian who was saying goodbye to Kurt.
When their lips met, both boys felt their hearts skip a beat and pulled back after a few seconds, blushing slightly. As Sebastian rubbed the back of his neck, Kurt looked over his boyfriend's shoulder to his step-brother and gave him a wink and grin.
His plan worked. Operation: First Kiss was a success.