That's your opinion; what's morally acceptable behavior in society is determined by the majority, and I'm not so sure you'd have the majority on this one.
10 years from now, if this site's still up, someone will revisit this thread and laugh at the uproar a few private pictures between adults caused.
Also, my parents started dating when she was 20 and he was 30; luckily her parents weren't as overbearing as janny & tornado
If his contract creates an at will employment status for the first three years, the guy will have no standing in court. Personally, if my 17 year old went to his school and I heard about this, I'd want him gone too. He did nothing illegal, but I hold teachers to a higher standard of behavior. As for the majority argument, I don't think that even close to a majority of Americans engage in this type of thing. I think it sucks that his girlfriend didn't take better care to keep this message private, but then again she's only 20 and 20 year olds don't always make the best decisions. He has no right to privacy argument because his privacy wasn't invaded, the pictures were taken from the girlfriends phone. She MAY have a privacy argument against her mother depending on who's property the phone is and other factors that create an expectation of privacy. The problem there is what are her damages? Without provable damages, there is no lawsuit.
I don't think that this guy will get his job back and he likely won't teach again anywhere close to there, but the school may offer some sort of settlement for this to go away. This should be a good lesson to everyone. NOTHING IS PRIVATE ONCE IT LEAVES YOUR PHYSICAL POSSESSION. If you want to keep your sex life private, keep it behind closed doors. There are just no winners in this situation.