That part in Three Worlds Collide is intended to show that future humanity is as far removed from us as the Baby-Eaters and the Superhappies are from future humanity. It's not put forward as an example to follow. Shock and disgust are the intended reaction. (Also as a sidenote, I remember Yudkowsky saying that he probably should have put in a different example.)
I can't comment about rationalists and BDSM since I'm not a part of the latter community. What I do know is that subs tend to be just as happy with the arrangement as doms are and that a sub not being happy with the thing is seen as a bad thing.
Also, BDSM isn't just about inflicting (or receiving) pain for sexual pleasure. Aftercare is an important aspect of the relationships, from what I read.
Previous Open Thread: http://lesswrong.com/r/discussion/lw/k94/open_thread_may_19_25_2014/
You know the drill - If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
Notes for future OT posters:
1. Please add the 'open_thread' tag.
2. Check if there is an active Open Thread before posting a new one.
3. Open Threads should start on Monday, and end on Sunday.
4. Open Threads should be posted in Discussion, and not Main.