Do you support the legalisation of polyagmy and polyandry

[The Grand Nudger]

But why justice and fairness?

Because we've assigned a positive value to these things.

And why not other values, like force or authoritarianism

Similar to the above but in the reverse.

or survival of the least fit?

We do sometimes draft laws meant to favor those we see as being particularly vulnerable. "Least fit" probably doesn't accurately describe this, but I like to pretend you aren't a troll when the subject is interesting to me.

Just because we get good feelings when we think about justice and fairness? Is it ultimately about the dopamine kicks in our brain?

If that's the reason we give for it (it usually isn't) and we all get into lock step then that would be why a law was created or adopted and then maintained, sure.

We need to have some rational basis for these positions.

Who says we don't? Plenty of rational basis for the adoption and application of law follows from the initial value judgement. "We agree that we positively value such and such, how then do we secure the safety of such and such and what do we do when the value of such and such is threatened"

If not, we are no different than the religious, and the religious have equal right and are afforded equal sway in our society.

They do, and they are. But don't be so quick to jump the gun. The difference between say a secular law and a religious law is the invocation of authority and subsequent explanation from that authority.

"Fairy tale says so, because fairy tale says so"
vs
"We say so, because we value such and such"

-Why- we assign a particular value to any given thing is always up for discussion, and this might help to explain why laws change as values change, eh? But that would be entirely too obvious, wouldn't it......

But hey, why justice and fairness indeed? Why do you value justice and fairness (if you value justice and fairness)? I personally value justice because it insulates me and mine (and yours) from things that I assign a negative value to, greases the wheels of civilization, as it were. I value fairness because I see it as a prerequisite of justice, what justice could there be without fairness or equality (fair being part of some definitions of justice, of course). Now, being human I'm personally willing to bend on either. I can imagine a person for whom I would withhold justice, a person for whom I would withhold equality....but on the balance of it I realize that if I could do that to someone else then they could just as easily do it to me, and that my own personal ticks probably aren't sufficient reason to decide who should get justice or fairness all by their one-sies.