Suggestion: learn a lesson and move on.
Nothing in the law requires the employer to have had an existing specific rule that applies to your conduct. The prohibitions in the state penal code are quite sufficient and your employer may rely on that body of law in defining misconduct. If you got caught, by definition your conduct was not sufficiently attenuated from employer premises, staff and activities.
Figure out what's wrong with this picture for your own benefit in the future, and move on with your life.
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I agree with Ms. McCall's ultimate conclusion regarding the state of the law.
Assuming that you and your co-worker were "at will" employees -- i..e that you had no employment contract -- you could have been terminated for any (non-discriminatory) reason, or for no reason at all. The reason they actually gave was not an impermissibly discriminatory one, so they were within their rights to terminate you.
I also agree with Ms. McCall that you should both just get on with your lives. However, I decline to further moralize. You did something unwise which a very sizable fraction -- and perhaps even a majority -- of Americans have also engaged in at some point in their long lives. You should just remember that these days Little Brother is always watching.
As an editorial aside, the good news is that, if instead of oral sex in a semi-private setting, you had instead wanted to maintain a collection of powerful firearms in your vehicle while it was parked on company premises, a growing number of states (16 as of this writing, though not yet California) would have had your back, and prevented your employer from "firing" you for this (pun intended). Go figure.
THIS POST DOES NOT CONSTITUTE LEGAL ADVICE, DOES NOT IMPLY ANY ATTORNEY-CLIENT RELATIONSHIP, AND IS PROVIDED FOR GENERAL INFORMATIONAL PURPOSES ONLY.
First, to clarify, you can be fired for almost any non-discriminatory reason, and termination under the circumstances you describe is ordinarily legal.
A separate issue entirely is your right to unemployment benefits. In general, a claimant will be disqualified if they are terminated for "misconduct." The EDD has written extensively about what is considered misconduct here: http://www.edd.ca.gov/uibdg/Misconduct_-_Table_of_Contents.htm I'd take a look at these materials for suggestions on how to make your case to the EDD. Hint--it may be helpful to argue this occurred off company property and off the clock.
This answer is a general interpretation of the law and is not fact specific to your case. Likewise it does not create an attorney-client relationship. You should seek an attorney for a review of your specific facts and documents.