Lemme have another go at this.
If there are different laws for different groups, then the differences between the groups will grow. People will join Camp A or Camp 5 or Camp Potato, and their actions will depend upon what camp they are, and are not, in. Even those—especially those—who don’t care one whit about camps will be pressured to choose, to pick a side.
Absent a neutral law, neutrality is hard to maintain.
And absent neutrality, pluralism is hard to maintain.
AbsurdBeats 
theology aside is there really any thing like a rule of Law? don’t we really just have texts, customs, people, etc, and if so how uniform can a code be?
Well, yeah: the Civil Rights Act of 1964 which required public accommodation—that was a fine piece of positive law.
And that’s what I mean (in this case) of one law for all: if you’re a business in the business of serving the public, you hire/serve all comers. Does it always work in practice? Of course not, but it sets a standard which, in so setting, creates expectations of equitable treatment.
I in no way discount the force of culture and habit—in fact, it is precisely because culture and habit can be so strong that a law is required to break ’em down.
http://www.wncn.com/story/28664509/first-woman-in-us-sentenced-for-killing-a-fetus#.VRr1fYhyK