Tim Stanley points out in both that they were not going to debate whether abortion should be legal or not:
this wasn’t a pro-life demo and the subject wasn’t whether or not women should have the right to choose abortion. Even though I was speaking for the proposition, my speech would've begun with noting that the motion has nothing to do with abortion rights per se and was simply a consideration of how having effective abortion on demand affects wider society. Brendan, speaking for the opposition, would've doubtless done a fine job and probably run rings round me. It was a fair and free debate that I half expected to lose.
The Catholic Herald now has a statement from a barrister: Christ Church may have broken the law:
A barrister has said that an Oxford college’s decision not to host an abortion debate is unlawful.
Neil Addison, national director of the Thomas More Legal Centre, said: “It’s an unlawful decision under the Education No 2 Act 1986, which guarantees freedom of speech in universities.
Authorities at Christ Church, Oxford, ruled this week that Oxford Students for Life could not hold a debate on the motion “This House Believes Britain’s Abortion Culture Hurts Us All” at the college. The decision followed calls by the Oxford University Student Union’s Women’s Campaign (WomCam) to cancel the debate between journalists Tim Stanley and Brendan O’Neill.
You might note the barrister's organization: The Thomas More Legal Centre:
We are a predominantly Roman Catholic Organisation in origin and ethos but we offer our services to all Christians in support of shared Christian principles and faith
We are also concerned about any attack on the Christian heritage of England and Wales by attempts to remove Christian symbols or prevent the carrying on of Christian traditions. We are willing to support legal actions to prevent the destruction of the Christian heritage of our Island
We take our name and inspiration from Saint Thomas More the English Lawyer and Lord Chancellor who was martyred in 1535 because he refused to submit to a Tyrant or to compromise his Catholic Faith and principles.
St. Thomas More, pray for us!