I sort of agree with you in that I think the highest quality of commenters won't be super squicked out by either horse sex or talking about cranial capacity differences between races or whether a trolley should run over 3 babies or 2 toddlers. But that's too high a bar! I think plenty of people are disgusted by certain topics and it's worth acknowledging that even smart, mostly well reasoned commenters can be scared away or simply feel unwelcome. Reddit can be a home to various mutually hating subgroups because of its structure, but we use a much less divided system and it's worth considering what topics attract what sorts of commenters.
Plus I don't actually want to talk about horse sex all the time as a filter.
A long blog post explains why the author, a feminist, is not comfortable with the rationalist community despite thinking it is "super cool and interesting". It's directed specifically at Yvain, but it's probably general enough to be of some interest here.
http://apophemi.wordpress.com/2014/01/04/why-im-not-on-the-rationalist-masterlist/
I'm not sure if I can summarize this fairly but the main thrust seems to be that we are overly willing to entertain offensive/taboo/hurtful ideas and this drives off many types of people. Here's a quote:
The author perceives a link between LW type open discourse and danger to minority groups. I'm not sure whether that's true or not. Take race. Many LWers are willing to entertain ideas about the existence and possible importance of average group differences in psychological traits. So, maybe LWers are racists. But they're racists who continually obsess over optimizing their philanthropic contributions to African charities. So, maybe not racists in a dangerous way?
An overly rosy view, perhaps, and I don't want to deny the reality of the blogger's experience. Clearly, the person is intelligent and attracted to some aspects of LW discourse while turned off by other aspects.