You have the right to defend yourself, but since Missouri is an employment at will state, the employer can terminate you for doing so as the law stands right now.
The "employment at will" relationship is very much in the employer's favor, although there are some exceptions like you not being able to be fired due to race, age, gender, etc.
There is another exception known as the "public policy exception". This exception is the one used for whistleblower cases.
I think you would have a good argument to make that defending yourself against sexual harassment fits under the public policy exception and thus the employer shouldn't be allowed to terminate you for this.
However, as I mentioned earlier, the law hasn't looked at a set of facts like yours yet and so you would have to be prepared to find a lawyer and then go through all of the steps such as filing with the EEOC, allowing them to investigate, then if they choose not to sue (and they rarely sue) using your own lawyer to do so.
You don't mention if you are a woman or a man, and I don't think that at this point it makes a difference for a lawsuit, except that if you are female you can also claim gender discrimination.
You can find a lawyer to assist you by going to www.lawyers.com and in the section for Area of Practice enter Civil Litigation or Employment Law. Either of those will have the skill set you need.
If your question has been answered then I'd offer my best wishes to you and ask that you please not forget to leave a Positive Rating so I receive credit for my work.
Of course, please feel free to ask any follow up questions in this thread. I want to be sure that all of your questions are answered.
I may have to leave the computer for a little while but if so I will be back in an hour or so and we can continue if you have follow up questions.