A professor sued Marquette University, a Catholic school in Milwaukee, Wisconsin, for suspending him and then terminating his tenure in 2014 over a blog post questioning an instructor's stand in favour of gay rights.

Prof. John McAdams and the Wisconsin Institute of Law & Liberty (WILL) filed the lawsuit on Monday before the Milwaukee County Circuit Court against the university for breach of contract for suspending him and banning him from the campus.

McAdams' suspension stemmed from a blog post he made in November 2014 in which he presented an incident between Marquette instructor Cheryl Abbate and an undergraduate student over gay rights.

Abbate told the student that when it comes to gay rights, "everybody agrees on this, and there is no need to discuss it." However, the student disagreed.

"In this class, homophobic comments, racist comments, will not be tolerated," said Abbate who then told the student to drop out from the class.

When McAdams wrote about his incident on his personal blog, Marquette censured and suspended him despite the fact that the university guarantees its tenured faculty academic freedoms including the right to free speech, according to the institute.

Last February, Marquette President Michael Lovell said he would follow the recommendation of the faculty hearing committee to suspend McAdams without pay until January 2017, adding that the professor needs to apologise to get reinstated.

McAdams has refused to apologise.

"I have spent nearly my entire career at Marquette University. I am proud to be part of the Marquette community and I have used my voice to both defend and criticise the university to ensure it holds to its Catholic traditions," he said.

He said the most overlooked aspect about the gay rights incident is that no one in the university administration took seriously the complaint of the student.

"For blogging and defending an undergraduate student, Professor McAdams is being suspended," WILL President and General Counsel Rick Esenberg noted.

The lawsuit asks the court to issue an injunction to reinstate McAdams as a tenured professor and for the University to pay unspecified damages.

Marquette argued that the issue is not about freedom of speech or academic freedom but McAdams' conduct toward Abbate, a university instructor and a former graduate student.

"Where Dr. McAdams crossed the line is when he launched a personal attack against a [former] student [Abbate], subjecting her to threats and hateful messages. Dr. McAdams continues to use the [former] student's name on his blog, leading to more hostile and threatening messages," the university said.

It also claimed that the case is not about McAdams' political views but about his actions towards Abbate.