Well-explained. It seems that you're saying that it's sometimes acceptable to infer policies and ideologies from the collective behaviour as a group. And on reflection, I agree. My real concern, I think, is that judgements like this, made from the outside, serve only to further polarise the behaviour.
For example, describing Reddit as sexist, as an outsider, has the effect of removing Redditors who do not want to be considered sexist (probably because they're not), which in turn increases the net-sexism of the community. Labelling a community from the outside may help to shun them from wider society, but it doesn't help to fix them: that has to happen from within. (not that I know the best strategy to do that with Reddit and sexism, though, although I try to do my part to downvote/criticise/educate where appropriate)