The minute you allow intolerance to exist, you have lost.
Karl Popper framed it incorrectly. He makes it sound like a moral imperative and the hateful have seized on that.
If tolerance is a moral issue there is little that can be done about those who choose to be intolerant. Being moral is an individual choice.
It IS a social contract, howevet. If I want my lifestyle to be tolerated then I need to tolerate that of others, unless my (or their) lifestyle imposes unnecessary restrictions on the other.
For example, all the Christians whining about Christianity not being tolerated. They're getting it wrong - nobody is saying don't be Christian. They break the contract by imposing their specific values on everyone else by force.
There is a baseline on which the vast majority agree concerning appropriate behavior in society. Murder, for example, is generally considered wrong. We don't tolerate it.