There seems to be a misunderstanding that the opposite of forbidden is mandatory.
In some parts of the world, it is mandatory for women to wear funny clothes. This is bad. So to counteract that, other countries are moving toward making it forbidden for women to wear funny clothes. This is almost certainly bad as well, but more importantly it's wrong-headed. Forbidden is not the opposite of mandatory. After all, forbidding alcohol is the same as mandating temperance.
Another example of this rankle showed up in the great Draw Muhammad kerfuffle. Some people have the silly idea that it is forbidden to draw pictures of Muhammad. Other people who do not hold this silly idea had their own silly idea: "the opposite of being forbidden to draw pictures of Muhammad is to draw pictures of Muhammad." In fact, the opposite of being forbidden to draw pictures of Muhammad is not being forbidden to draw pictures of Muhammad. Fortunately for me, I'm not forbidden to draw pictures of Muhammad, so I've won my victory against a silly idea without even doing anything.
More recently, PZ Myers got on my last nerve in a post about Paul Kurtz, whom PZ takes to task for being 'soft' on blasphemy. Quoth PZ, "You have not protected the right to blaspheme if you also gag yourself and say you won't." So apparently the way to combat the idea that blasphemy is forbidden is to make blasphemy mandatory. On this point I took him to task by noting that his position would make it impossible to protect the right to have hot gay sex without having hot gay sex. If mandatory were the opposite of forbidden, I would have no choice but to have hot gay sex.
There is a word that embodies an important liberal (and American) ideal; that word is 'allowed'. The opposite of mandatory/forbidden is allowed.
I am allowed to wear funny clothes.
I am allowed to have the idea that I must wear funny clothes.
I am allowed to have the idea that I don't have to wear funny clothes.
I am allowed to draw a picture of Muhammad.
I am allowed to not draw a picture of Muhammad.
I am allowed to blaspheme.
I am allowed to have hot gay sex (if Dr. Pookie agrees)
There are good reasons that laws mandate or forbid certain things, but there is plenty of room for allowance.
ETA: Donovan reminded me that, despite my near-total ignorance of political philosphy, I wanted to end this screed by quoting a political philosopher who wrestled with these very issues and came to a similar conclusion:
Kang: Abortions for all.
Very well, no abortions for anyone.
Hmm... Abortions for some, miniature American flags for others.