I think that if you honestly KNOW in your heart that the kiss was a mistake and that it WON'T happen again and you sincerely want to work on your marriage then it's best not to tell your husband.
The amount of permanent damage that might be done by telling him is probably not worth it. Telling him could cause him to stop trusting you, make him insecure, etc. and if you are SURE that it was a mistake and won't happen again then I say there's no point in putting him through that just to make yourself feel better.
Now is the time to really take a good look at your marriage and put your all into making things better.
Good luck!!
The amount of permanent damage that might be done by telling him is probably not worth it. Telling him could cause him to stop trusting you, make him insecure, etc. and if you are SURE that it was a mistake and won't happen again then I say there's no point in putting him through that just to make yourself feel better.
Now is the time to really take a good look at your marriage and put your all into making things better.
Good luck!!