I do get some cult-like vibes from the Yudkowsky part of the rationalist community, but I think calling it a "sex cult" is going way too far unless there's some hard evidence. It's a community that has many polyamorous and kinky people in it, and many people who have been or are doing communal living, whether because they want to or because of Bay Area rents. That doesn't make it a sex cult. I've never seen any evidence that Yudkowsky, for example, is exploiting his status in the community and his minor degree of fame in order to obtain sex any more than the average person who has a bit of status and fame uses it to obtain sex. I'm not aware of any evidence of coercion or brainwashing. There have been some allegations of sexual abuse in that community, but from what I can tell no more than would be expected for any community of that size.