Since you raise it as an example: in the UK opposition to gay marriage (and a few years earlier, more basic gay issues) didn't melt away through empathetic discussion or addressing their concerns but simply from the government going ahead and doing it (I doubt they'd have gone to war for it if it were necessary, but that underlines the point about might being more significant than right in the debate). Public opinion tended to shift more when decisions were made and the debate stopped.

One of the first things to realise about people's apparently very strong views is that often they're simply echoing what they think is the majority view (but they generally really won't like you pointing that out). A second is that often they change quite swiftly when people stop talking about it. Not only are they not constantly being reminded that X is bad by people and sources they trust, but they also don't feel obliged to keep defending a position they held a couple of years ago when not regularly prompted to justify it.