Actually, the ONLY time people say that is when their particular vice (drugs, prostitution, homosexuality, etc.) is the subject of legislation.
Well, I never say that, and I have none of those vices ... but the kernel of truth in that sloppily-phrased expression is that one can't successfully legislate against those immoral acts whose only participants are consenting adults. The only demonstrable result of such efforts is to enrich criminals.
Homosexual "marriage" is another thing entirely, since marriage is a positive act by an institution (government or house of worship).
Those other things such as drugs, prostitution, homosexuality, etc., have been legislated against and controlled under the rationale of ‘good of the culture’.
In my mind, it’s a valid argument. Healthiness/Wholeness is a legitimate cultural concern. We forbid blood donations from those in countries with mad cow disease even though the probability of infection is in the multi-millions to one.
The “consenting adults” criterion is the poisonous core of the Leftist’s ideology, although they are hypocritical in applying it (non-discrimination laws making unwilling servants of private individuals, etc.) There is demonstrable societal harm done by the actions of adults who consent to engage in immoral behavior.
Would you have prostitution de-criminalized? All drugs? Polygamy? Public nudity and obscenity?