I think that they should be fired!
This is not an excusable offence.

If I were an employer, I would try to think of how this could affect the work environment later on down the line, and if it would create an uncomfortable environment for my employees. As adults, we need to make wise decisions and doing it on the job ain't smart!!

You have to think about whats important...and since they (the acting parties) decided that getting off was more important than getting the job done, then they become expendable. Thus, firm disciplinary action should be taken.