what's the evidence that polyamory has 'harmful effects... on societies, like encouraging male violence and suppressing womens rights'?

There is lots - I don't want to exhaustively list it for you, but you can easily google to find stuff like this economist article about the negative social effects of rampant non-monogamy.

It seems a bit patronizing to say that vulnerable young women can only understand that something is going wrong if the guy who hits on them is married.

It would be patronizing, which is why I didn't say 'can only'. Please don't misrepresent me like this.

What is the case is that some victims don't understand that something is going wrong, and it is good to make this clearer. Also, bystanders who could intervene often can't tell something is going wrong - it would be good to make this clearer also. We do this in many other areas - to avoid moral violations that are hard to catch, we insist on easily-observable bright lines even if those bright lines seem less inherently morally noteworthy. As you point out, this can make it easier for victims to speak up, because the violation is more unambiguous.

they're not (usually!) tempted to mock or scorn something just because it's weird or unusual

I'm not saying it is bad because 'just because it is weird or unusual', nor was I scornful or mocking. Rather, I made specific arguments for why the practice is bad. Just because we tolerate weird and unusual things doesn't mean that bad things are ok so long as they are also weird and unusual.

In another comment in this thread you literally express an opinion on other people's consensual relationship choices!

Thanks for the thoughtful response!

I think you may have the sign wrong on this though:

especially when poly people often face hostility from the rest of society

In general people have decent reasons for the things they do. For this reason, EAs generally align with most of western society on most issues. e.g. we are against theft. The fact that the rest of society is also hostile to thieves isn't a reason for us to be nice to thieves - it is supportive evidence that we should also avoid theft, because similar reasons apply. Unless there is some strong EA-specific argument at play, I think our default in most scenarios should be to adopt similar norms to the rest of society.

It's also worth noting that while western society is generally somewhat intolerant of polygamy, much of the world is not. It is legal in much of Africa and Sourthern Asia, and quite common in some countries. However, I generally think we should prefer to adopt western moral norms to those of these places, partly because they often treat women poorly.

Yes reducing workplace and social harassment of women is an important issue for inclusivity. I brought this up because there is a lot of research that monogamy is good for women because it reduces violence and increases wellbeing.

The rhetoric around "senior men" and "the leaders wives" rings very handmaid's tale-y which is probably an exaggeration. 

Do you deny that most organizations are lead by senior men, who sometimes inappropriately approach more junior women? Or that traditionally most senior men had wives? I don't understand the handmaiden's tale reference. In that book important men get multiple wives which I am opposed to?

Also not sure why the solution in the third paragraph isn't "don't hit on women who are your professional junior."

The same reason the solution to theft isn't "don't steal". We need a response which is robust to some bad actors, not just assume everyone will be good. This helps increase the social costs of bad behaviour.

Just to flag in some ways it would be good to be less inclusive. There is a lot of discussion right now about how/if/when we could have spotted the FTX/crypto fraud sooner and that is all about being quicker to exclude them.

To engage more productively with the prompt, I think the de-normalisation of Polygamy seems plausible. I've long been uncomfortable with (some) EA's embrace of this, as given the harmful effects of the institution on societies, like encouraging male violence and suppressing womens rights. Even though these issues didn't seem like huge issues for EAs, I don't think we should adopt norms that would be bad for society if everyone did them. But there seem to be also two significant more concrete inclusion reasons to oppose it.

Firstly, it enables predatory men and abuses of power. In a traditional environment, all the senior men will be married, and thus any proposition they make to vulnerable young women is clearly illicit. It can still happen - though probably with lower frequency - but the woman will clearly understand from the beginning that a norms violation is occurring, and there is more support for shutting it down sooner. Additionally, to the extent the leaders wives are involved in the community, there is a native constituency naturally opposed to this behavior.

Additionally, as people have pointed out, sexual relations in the workplace create clear conflicts of interest. There is a reason they are tightly regulated in many professional environments. This is not the first scandal we have had where key decision makers seem to have covered up for their romantic partners.