How come adultery is a firing offense for military officers but not for the Commander-in-Chief (WJC).
Because the Commander in Chief is not under the Uniform Code of Military Justice - nor do we want him to be. I am a PROUD veteran, and I don't want my President to be under military jurisdiction. We have damn good reasons for having civilian control of the military.
While it does occasionally lead to issues like the one you noted, the alternative is unimaginably worse.
Your Senators should answer that question.