I think with our current understand of neurobiology & psychology we can pretty well understand on average which actions can cause harm.The problem is, morality and harm are both incredibly hard things to define which is one of the major problems with deciding what should be illegal and what shouldn't be.
Obviously nothing is black & white & requires rational thought - just because we can have an objective morality - it does not have to be an absolute morality.
It's just objectively judged based off a number of considerations with applied reason & consideration for the subject matter.
Stealing a banana in my personal opinion is not always immoral, it's illegal - there is a difference, the act of stealing a banana does not always cause harm - neither is it immoral if it's being done by a person to prevent greater harm (starvation of a child as a example).For example, stealing a banana from a shop is clearly immoral yet barely harmful and carries the possibility of a criminal punishment.
What's immoral is having a society structured in such a way that an individual is put into a situation in which they have to steal a banana.
If a person was never put into a situation where they had to steal one & still did, they would either be trying to cause harm by disrupting somebody's business, or a kleptomaniac - I'd also argue that both of those would be symptom's of underline psychological conditions which would require curing, not punishment.
I believe this is more of a relic of how we structured our civilisation with the rights of women on the bottom & men on the top - in the past & in many cultures today it WAS a crime for women to commit adultery - while men were able to avoid this due to being in the position of power.Yet adultery, which is arguably more immoral and certainly more harmful is not a criminal offence. Despite this logical inconsistancy, it seems that this is justifiably 'the correct way' of dealing with things.
If we used an objective framework, deceiving another & constantly cheating on them is indeed worse than basic stealing.
But the brain is a physical object, one which is quite well understood, as human behaviour well understood & predictable/documented - it's not that much of a stretch to make a case for objectively harmful actions.The concept of harm is wooly. Unless that harm is easily quantifiable (which is only really true of physical harm as financial harm is relative) then the harmfulness of an action is dependent on the sensitivity of the recipient, which actually turns 'harm' into 'offence'. Harm and morality in its entirety is thus entirely subjective.
Offence needs to be judged based on the actions,
If person A takes great offence to two gay people being in a relationship - then person A's offence needs to be judged off a logically consistent framework.
Person A would need to justify why there offence should be taken seriously.
Firstly, person A is not abiding by the "do unto those" rule, as they would not like somebody telling them to engage in a relationship with a member of the sex different to there default preference.
Secondly, they are also offended over something which objectively causes no harm.
On that basis the "offence" of person 1 would rightly be dismissed of the crying complaining of a bigot.