Basically turning morals into laws. e.g making gay acts illegal.
A legal moralist believes everybody should behave as they wish, but no two legal moralists can agree.
Justice - right or wrong - is generally agreeable But legal moralism is not generally well agreed.
The response is democracy can decide what should be morally legal, but that is avoiding the issue, there might be an outcome, but it is not stable as winners must fight off losers.
In any case what makes the majorities vote the right outcome?
Both assume social justice and legal moralist assume a gods eye view. Trying to create a heaven on earth. (making up for where god is not doing things)
To adopt one social justice or moral code you have to reject every other one. Means people must live in a society is immoral to them and always competing war of morals.
These people ideally want their "religion" imposed, but everyone's second best option is not to have another religion imposed on them, meaning it is the most stable solution. (where secularism came from)
In the same way that religion is a matter of conscience, so is moral decisions. Your moral ideas does not violate anyone else's.