Yes they did, but then political pressure rather explains that, doesn't it, for how many MPs would want to vote against a referendum? And, in any case, as we've pretty clearly seen before and since, the referendum was called not to resolve the question but to squash it -- otherwise, there would have been far more preparation for the actual outcome than there was.
Even leaving all that to one side, though, it is -- or, at least, should be -- possible to start out on a course, realise that this course is hopeless and damaging, and therefore change tack. There's no obligation to carry a policy out to the bitter end if it's causing too much damage. Sometimes persevering is worth it, sometimes it is not; but, no matter what side of that divide we stand on, it would be nice to at least agree on *that*.