You have the right to be homophobic -- but not to put these beliefs into harmful action.

An Englishman's home is his castle, or so the phrase informs us. That small scrap of land that is ours to do with as we please. However, as Christian guesthouse owners Peter and Hazelmary Bull have found, once said castle is opened up to the public, the rules begin to change. Specifically, if you say you'll rent your rooms to strangers, it's illegal to turn away the ones that are gay.

Nelson Jones wrote this week that "the intimate circumstances of bed-sharing...complicate the situation". I would have to disagree. Banning gay guests from your premises becomes no more legal if the rule "only" applies to those who might end up having sex. Or in this particular case, those who wish to do so in a double room and without one of the couple making a walk of shame to spend the night back in their single bed.

Though some of the issues raised by this trial may be complex, the concept of discrimination is not. Just as it's against the law to run a business and only serve people with white skin, so it's against the law to run a business and only serve people who like to sleep with the opposite sex. That the banned customer could go elsewhere does not, as Nelson suggests, change this. There could be a hundred other guesthouses available to a gay couple but it would have no relevance to whether it was right or legal for one to turn them away.

Everyone (conducting themselves within the law) has the right to be served everywhere, and to say a policy like the Bull's "need not unduly inconvenience gay couples" is to severely reduce what's wrong with discrimination.

When civil partners Martyn Hall and Steven Preddy were denied a double room, the harm didn't simply come from the effort of re-arranging their plans, or even the (at best) embarrassment that such a need would cause. It came from being excluded because of a biologically determined difference, from being banned from doing something because of who they are. The law says this is wrong. That "the God worshipped by the Bulls does not" is, though unfortunate, irrelevant. Discrimination is discrimination, whether it stems from the playground or a Holy Book.

It would be easy to see such a verdict as an attack on freedom, an attempt by the state to take an unpopular belief and make it illegal. This would, though, be inaccurate. This is not a case that judged the right to be homophobic (or "old fashioned" if that is what we wish to call it). It is a case that judged the right to be homophobic and use that belief to hurt someone else.

How hurt is defined is fundamental -- whether we live by the notion that prejudice only hurts its victim if it involves blood and a physical blow. Nelson is right that philosophy can teach that "multiple preferences" are best, provided they don't cause ill-effects, but it can also tell us the point at which these ill-effects mean our actions must stop. Liberal theory -- the ideas we base our laws on -- sets clear restrictions on personal liberty: "The right to swing my fist ends where the other man's nose begins." The Bulls have the right to think homosexuality is wrong. If they so wish, they have the right to be repulsed by the thought of two men having sex and even to declare out loud the perils in this sin. They do not have the right to put these beliefs into harmful action, to use them in a way that leads to discrimination.

No one laid a finger on Martyn or Steven. By all accounts, Mr and Mrs Bull were very polite in telling them they were not allowed to share a room with the person who is their partner by law. This does nothing to change the fact this was discrimination. One can't help but wonder whether if their reason had been something other than sexuality, this would even be under contention. There would be unanimous disgust at a guesthouse that held a policy of "No Blacks with Whites Allowed" -- and that it involved "the intimate circumstances of bed-sharing" would evoke little sympathy if inter-race couples were told to take separate rooms.

Such beliefs, in these times, cannot be put into practice. If you open your castle to the public, it's the price you have to pay.

Frances Ryan is a freelance writer and political researcher at the University of Nottingham. She blogs at Different Principles and tweets @frances_ryan