It's possible you're thinking of:
* A different Ben 10 series: There are several Ben 10 series, including Ultimate Alien, Omniverse, and the newer reboot. Maybe the kiss you're thinking of happened in one of these series?
* A fan-made work: There are many fan-made stories and videos for Ben 10, and it's possible you saw a kiss in one of these.
If you can provide more details about the episode or the series, I might be able to help you find the specific episode you're looking for.