You seem to be saying "[T]he law that is supposed to be the written, clear representation of the moral behind it. If you want to challenge [a given law], first you have to [change that law to recognize as valid the point that you wish to talk about].". Am I wrong?
Then, your statement can be better phrased as "Illegal things are always illegal for a good and proper reason. It's improper and non-productive to discuss whether or not the things that they make illegal should be illegal.".
This is an indefensible statement.
Here are a few relatively harmless, or -in some cases- everyday things that are or were illegal in parts or all of the US:
* Oral and anal sex between unmarried persons (known in the state as "Deviate sexual intercourse") is currently (at the time of this writing) illegal in the State of Alabama. [0] What is "Deviate sexual intercourse"? "Any act of sexual gratification between persons not married to each other involving the sex organs of one person and the mouth or anus of another." [1]
* Polygamy and polygyny are illegal in the US. [2]
* Until recently, gay marriage was illegal across the entire US. Now, it's just illegal in significant parts of it. [3]
* Less than a hundred years ago, it was illegal for women to vote in the US. [4]
* Marijuana consumption and production is still very illegal in much of the US, and is considered by the Federal government to be just as bad as heroin, crack, and cocaine consumption and production.
Bad law gets made all the time. What's more, the values of a population tend to change more rapidly than the laws that govern their behavior. Bad, or no-longer-useful laws continue to be enforced for ages before they get struck down (if they ever get struck down).
One critical part of the process that prunes useless law is earnest public discussion of the applicability of the law. Silencing earnest honest discussion of the form "If this illegal behavior actually causes no harm, should it be illegal?" is troubling and harmful, as this sort of discussion is a huge part of how improper laws are removed.
[0] http://alisondb.legislature.state.al.us/alison/codeofalabama...
[1] http://alisondb.legislature.state.al.us/alison/codeofalabama...
[2] http://en.wikipedia.org/wiki/Legal_status_of_polygamy#North_...
[3] http://en.wikipedia.org/wiki/Same-sex_marriage_in_the_United...
[4] http://en.wikipedia.org/wiki/Nineteenth_Amendment_to_the_Uni...