This statement generally comes up when one is giving their opinion in one fashion or another about morality. They are saying what they think is right or wrong and how that ought to play out in society. This self refuting statement comes up often when a Christian is expressing their viewpoint that goes against the grain (as it often does in our morally bankrupt society).
This challenge is self refuting. It defeats itself because it is a challenge to you that you ought not force others or ask others to live according to your moral point of view. Yet that challenge itself is a moral point of view that someone else believes in and is asking you to live by. To put it simply, they're forcing their morality on you when they say that you shouldn't force your morality on someone else.
Everybody has a moral point of view that we are seeking to have believed and acted upon in society and we are seeking to do that by some measure. Take the homosexual marriage agenda. They are advocating that their moral point of view (gay marriage) is legitimate and should be accepted. Is that a moral viewpoint that should be "forced" on everyone? How legitimate is it to force that moral viewpoint? I won't enumerate here why that viewpoint is not a legimate viewpoint to be forced on society at large, but I do take exception to how that viewpoint is being implimented on our society, which in essence, is the "rub" that opponents of gay marriage have against gay marriage inter alia.
There are two ways to force your morality. One way is to compel or encourage people to act in a way you think is right and for the common good. Generally we use the force of social approval and disapproval to effectuate this method. Another way, the way of the gay marriage activist is to strong arm society with forceful measures- namely by force of law and/or the courts. This brings up a whole new topic of judicial activism, which can be discussed at another time, but a tactic that gay marriage activists are capitalizing on currently.
But one way another, EVERYONE is forcing a moral view on someone. If you are a moralistic person, shame on you.