The term "homophobia" is much abused these days. I remember a time, back in the 1980s, when it was actually a very useful word. It embodied a theory, widely believed in the gay community for at least a century, that those who expressed a violent disgust for gay people were probably afraid that they might be gay themselves. Or else they knew they were and were afraid of other people finding out. That was why it was called homophobia: the word phobia, after all, means fear and not hatred.
The concept of homophobia was packaged for wider public use by Stonewall in a very successful UK advertising campaign, in which various cliches about homosexuality were reworded to refer to homophobia instead. For example, one advertisement showed a man saying "My son's a homophobe. But I hope it's just a phase and that he'll grow out of it." The punch-line was always the same: "It's the prejudice that's queer."
This campaign challenged the popular perception that opposition to gay sex was simply a default position that did not need to be defended. It called on those who held that position to provide actual moral or theological arguments in favour of it, to prove that they were not just being homophobic. Since in fact many of them were just being homophobic and had no real defence to offer, the man on the Clapham omnibus was able to draw his own conclusions. It was an important turning point in the struggle of gay people for public acceptance.
Nowadays however, the word is increasingly used, not as a challenge to produce rational arguments in favour of one's position, but as a way of shutting down the argument altogether. Calling someone a homophobe has become very much like calling him a racist. Both words are convenient labels that mark an opponent out as an utterly despicable person whose arguments are not worthy of any serious consideration.
There is an old legal proverb: when the defence case is weak, abuse the plaintiff's attorney. The converse also applies: when someone putting forward a case is abused rather than answered, it suggests to onlookers that the abuser's case is probably rather weak. Nowadays an accusation of homophobia is less likely to make the recipient feel abashed than to convince him that he has actually won a moral victory.
This reaction is all the more likely because most of the anti-gay arguments come from deeply religious people who are already predisposed to regard themselves as martyrs. Martyrdom makes them feel good, and so those who bellow "homophobe" at them and maybe orchestrate a Twitter storm against them are actually giving them precisely what they want. Whereas what they need is to be given a proper explanation of why their arguments are wrong or their biblical interpretations misguided.
Having said that, there are still some genuine homophobes around. You can't really argue with the type of person who goes about with a banner inscribed "God hates fags!" That person is clearly not interested in offering any kind of argument; he is merely demonstrating violent emotions, which may well arise from fear of what lies within his own heart. Probably the best that can be done for him is to prevent him from working his feelings off on his innocent gay neighbours.
Physical or verbal gay-bashing can usefully be classed as homophobia and condemned on that basis. So, retrospectively, can the use of what seemed at the time to be reasoned arguments by someone who subsequently turns out to be a closet queen. A classic recent case is Cardinal Keith O'Brien, who became a self-appointed spokesman for Catholic opposition to gay marriage, only to be unmasked as a closeted gay bishop who had used his position of power to seduce some of his own priests. I think it is a reasonable guess that his stated position owed more to fear and self-hatred than it did to theology.
But let's not castrate a useful word by misusing it to refer to genuinely principled beliefs that we just happen to disagree with. After all, tolerance and free speech are classic Enlightenment virtues, and the ultimate test of our real belief in them is surely whether we extend them to the expression of opinions that personally offend us.