Robert Walenski, 49, who taught literature for 25 years in Cape Cod, was suspended last month and fired five days later for "conduct unbecoming a teacher". He admits to having produced eight soft-core adult films thousands of miles away in California.
"How does a teacher effectively teach Elizabeth Barrett Browning and Shakespeare's sonnets to a group of young women when he has this vocation and they know it?" asked Michael McCaffrey, the head of Mr Walenski's school district, who fired him.
Mr Walenski's lawyer, Roderick Macleish, said: "The conduct that took place took place privately and outside the school district and has no effect on Mr Walenski's role as a teacher." He said his client planned to take legal action against the school district.
The National Education Association is supporting Mr Walenski, saying he did nothing illegal.
In at least one earlier case, disciplinary action against a teacher accused of ethical misconduct outside the classroom was superseded by the teacher's right to free expression. In 1993, a high school reading teacher was reinstated after threatening a $1 million (Pounds 670,000) lawsuit against the school board that suspended him for disseminating racist literature in his own time.