It is an article by Roger Scruton under the title “When Hope Tramples Truth“. And here is the relevant paragraph which I completely endorse from my own personal experience:

People interested in truth seek out those who disagree with them. They look for rival opinions, awkward facts and the grounds that might engender hesitation. Such people have a far more complicated life than the optimists, who rush forward with a sense of purpose that is not to be deflected by what they regard as the cavilings of mean-minded bigots.

The issue, if there is an issue, is gay marriage but he is trying to make a larger point how when those with what he describes as an “optimistic” view of social change seize on some idea and declare it to be the truth, rather than testing that idea against all objections that can be raised, will instead persecute anyone who holds a contrary view:

It is easy to trace disasters, in retrospect, to the bursts of unfounded optimism that gave rise to them. We can trace the subprime mortgage crisis to President Carter’s Community Reinvestment Act of 1977, which required lenders to override all considerations of prudence and fiscal rectitude in the pursuit of an impossible goal. We can trace the current crisis of the Euro to the belief that countries can share a single legal currency without also sharing loyalty, culture and habits of honest accounting. We can trace the disastrous attempt to introduce responsible government into Afghanistan to the idea that democracy and the rule of law are the default conditions of mankind, rather than precious achievements resulting from centuries of discipline and conflict. And we can trace the major disasters of 20th century politics to the impeccably optimistic doctrines of Marx, Lenin, Mao, and the many others for whom progress was the inevitable tendency of history. Pessimism, so obviously vindicated in retrospect, is almost always ineffective at the time.

Caution is so much the watchword. A reluctance to depend on our reasoning powers alone, while tempering any conclusions we reach on the accumulated wisdom of the past, is the essence of the conservative temperament, and the only recipe for long-term survival in the hostile environment in which every civilisation must live.