There seems to be strong disagreement about my comment, so I'll explain why I believe it's somewhat arrogant for Effective Altruists to take a definite position in some relationship styles (certainly not all!):

(0) Like I said, this is a deep cultural issue, which evades many of the conventional tools of Effective Altruism. It doesn't mean we cannot discuss it, or even have personal opinions, but it seems that we should avoid taking position on it (as a movement), without consensus of society, given we probably lack the expertise and tools to make such judgement.

(1) Some relationship styles probably have conventional (contemporary) wisdom to cause harm to people. That includes abuse, or things that form consensus in social sciences to be harmful. I don't know much about customs around child marriage, but it seems like something that can be discussed, in light of cultural literature as well.

(2) It seems that polyamory is very much a cultural gray area, and I don't think there's any kind of consensus on whether it could be harmful and in what ways, or whether it could be good for individuals 

(reminder: things like this need to be seen from many points of views, not only through studies, but also from personal experiences that are very complicated -- think trying to justify numerically your favorite food. It's very difficult, it tends to evade conclusive and analytic evidence, instead appealing to intuition and maybe long discussions on taste and other factors that elude this kind of argument)

(3) This sort of evades from the core of Effective Altruism, that is to address most urgent and effectively actionable causes. I don't think policing relationship styles, around the community or not, or even minor cultural norms, is something we should focus on: again, unless we can back it from a social science consensus (and straightforward quality of life impacts), specifically because I don't think this will prevent suffering one way or another as much as focusing on our mission. 

I'd appreciate further feedback on those points or other points of disagreement.

I would say the relationship of a person is private, and it seems arrogant for us (Effective Altruists) to decide what relationship styles society at large should accept -- specially considering that we want to be welcome to all different cultures, from East and West, including indigenous cultures.

What should not be acceptable is any form of harassment, and it seems like a pretty good universal norm that Effective Altruism community gatherings and workplaces should be focused on that mission - EA. That's not to say relationships are completely banned and shunned, but it should be common knowledge that this is not what EA is for (finding partners) - and advice that it should be strongly avoided.  It should be clear what EA spaces are for (not purely for socialization, for finding partners, etc., but for helping others effectively and discussing how to achieve that!)

Note: unless there is clear consensual will from all parties and it happens outside EA of course - I don't think banning consensual relationships outright is wise or necessary.  Note2: I read a comment somewhere recently that 'You are allowed to ask people out at essentially all places, as long as there is immediate acceptance/consensus; however many places rightfully ban non-consensual approaches, i.e. rejected approaches. This may seem unfair, but it isn't since there are really many other places that allow people to meet each other and where the norms allow asking people out'.

We should promote a spirit of inclusiveness of all cultures and persons, and this probably requires establishing some norms around avoiding some kinds of behavior.

Edit: There seems to be strong disagreement about this comment, I'd appreciate clarifications. I might retract some things.