Mistake in slogan?

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In “To err is humane” (July 30), Dr Lim Chin Lam was uncomfortable with the sentence, “Does the family that steals together, stay together?” his objection being that “family” here is presented as a singular entity and therefore does not fit with “together”, the latter implying a joint activity between two or more entities. Reading it immediately brought to mind the slogan, “The family that prays together stays together” which I heard more times than I could count when I was at Catholic school. I know that the slogan has been universally used and I have, hitherto, never seen its grammatical correctness queried. For more on that slogan, go to en.wikipedia.org/wiki/Patrick_Peyton.