Ah I see now, thanks. I don't its nearly as common as "abortion is murder" though. Actually I'm quite embarrassed that I can't think of any liberal examples of the worst argument in the world with regard to social issues that people actually use regularly. I don't like any that I've seen so far.

The archetypical example of a struggle for equal rights has been identical political enfranchisement (e.g. the suffragette movement) or identical legal equality (equal right to property, etc) on an individual level.

It's not as different as it might seem. Initially, only property owners could vote, and so only letting men vote was effectively "one vote per household". With the "sensible" assumption that everyone is married and truthful with their spouses, giving votes to both spouses means either one candidate gets two votes, or they vote for different people and thus might as well have stayed home. Since the only difference there is that the folks counting the votes would have to do twice as much work (since spouses usually agree about everything), it would not be helpful to give every household 2 votes instead. People could have argued at the time (I'm not sure whether they actually did) that women do have the right to vote - the husband is voting for both of them (this is of course complicated by the fact that women could not own property).

Equal right to property works similarly - unmarried women could own property, but married women could not.