He makes a mistake when he says that 'tolerance means, literally, to engage with other people who are different'. Tolerance doesn't require an engagement at all but is the opposite of intolerance, that is allowing others to be different as opposed to wanting to stop them being different. Furthermore, the argument that modern liberal society says "we shall tolerate you as long as you pretend to be just like us" isn't true all or even most of the time. Most of the post-2001 critiques of multiculturalism have been along the lines that we have tolerated (that is, not been interested in) vast degrees of difference and this has led to social divides. However, that's not my issue here, as the question of tolerance will run throughout the piece on gay marriage.
What's interesting here is how he defines difference: 'how he or she is unlike me, in their faith, their ethnicity, their sexual orientation'. For me this gets to the heart of why the author just doesn't 'get it' as there are many people for whom these just are not relevant, but also show where other divides can be found in society that may be just as deleterious. I'd like to argue that what people consider to be an important difference says more about them or their conditions of life than it does about the existence of fundamental, necessary or inevitable divides.
However, in most of my own life and work I have been in places where faith/ethnicity/sexual orientation have been irrelevant, unimportant and not always explicitly articulated (and I don't mean because people feel a need to keep stuff hidden but because it's not at the centre of identity). However, the other identities which contribute to bringing people together may themselves be divisive.
For a long time I worked for a second-hand record shop chain. The staff were a mix of graduates and non-graduates, musicians and enthusiasts. They were from all around the world, lots of different 'ethnicities'. Some were gay, some straight. I presume some were religious but I'd guess that the vast majority weren't: it just wasn't relevant. Perhaps people kept their religion quiet, but I don't think many would have been freaked out by it. Indeed, spreading outwards across friends and acquaintances the same thing was true… most of us hung out with other people into the various music scenes.
In my research work I've found this time and time again with young people. Groups of teenagers and 20-somethings that are multi-ethnic and/or include people across sexualities, with no hint of racism or homophobia. Sometimes it might look like they are using the language of intolerance, but this can be the joking appropriation of what they see as the mainstream. Again, music is often at the heart of things: what defines the insider/outsider is whether they are 'in the know'. Youth cultures from the 50s onwards have often (not always) decreased the salience of faith/ethnicity/sexual orientation as what makes difference, bringing in new forms of difference.
While I'm out of youth culture, I'm still in a 'community' where these forms of difference are unimportant. I know that some of the people I spend time with are religious but we mostly don't talk about it. There are people from all around the world, and a few are gay. What brings us together are other identities – football, parenthood, jobs in academia. I don't talk to my atheist-with-Jewish-background friends about Jewishness, or my gay friends about being gay, my religious friends about believing. I don't assume that it's an absolute centre to their identity, I don't think they are representatives of a particular 'group'… there are other things to talk about instead. They are part of my 'groups' (i.e. academics/parents/footballers) and we talk about that.
And there's the rub. I largely stick to my groups too (except when doing fieldwork), and can see how I'll get to a point where I don't understand how people can live without thinking sociologically, without football, without music, or without spending time at kids' parties ;-)
Thus the reason that the priest just doesn't get it is because most of the time he's with other people like him. He's embedded in religion and even those non-religious people he talks to will see religion as a key part of identity (it's the current standard narrative). And he's less aware of those people who just don't see it as important.
A similar process is behind all the community cohesion debates: it wasn't religious difference that caused any Asian Muslim/white working class divides in Bradford, but once people are part of such a divide it becomes the divide that people define themselves by and there can then be conflict based on such identity. But we should remember that conflict can be based on any identity: those youth culture divides I mentioned aren't always peaceful. At one point it was the mods and rockers that were at war.