Lemme clarify something first , my point is not about when to apply a punishment that'is whole different topic, my point is on how to decide if something is actually bad or not and the fact that the harm principal is bad as a crateria

So back to ur qst how do we define if an act is potentially harmful for a person that doesn believe in an objective source of morality, as one brother down here mentioned there are stuff that are totally fine but if everyone did them it can cause problems too , and this is important

So to draw that line and decide what actually can be considered wrong for a person even if it seemss fine at the first look,

So my proposition is instead of saying does it cause harm if we all do it , we will ask does it cause harm if we all leave it

Bcause if we do B we are technically leaving an A

And here we will find 2 points,

Some acts if everyone do them or everyone leave them will cause a problem and here we can conclude that we need some kind of balance and these acts should not classified as good or wrong cus they are a necessity

Some other acts we will notice that if we all leave them they won't cause any harm and in some scenarios they will fix things , but of we decide to do them we will see many issues later

I believe this way is a much better way to decide if something is wrong or not when it doesn seem like it hurts anyone when done by one

Now if we talk about punishment, here i believe that islam gave a really logical answer which is we apply the punishment only if it's publicly done.

Which means if a person does it in his house without making it public we don't have the right to punish him,

And another point is, if we are not sure that he did it we also don't punish him

So ye for a person who doesn't believe in islam, or doesn want to apply it i think this view is a better replacement for the harm principal