Here's my attempt at explaining Eliezer's explanation. It's based heavily on my experiences as someone who's apparently quite atypical in a relevant way. This may require a few rounds of back-and-forth to be useful - I have more information about the common kind of experience (which I assume you share) than you have about mine, but I don't know if I have enough information about it to pinpoint all the interesting differences. Note that this information is on the border of what I'm comfortable sharing in a public area, and may be outside some peoples' comfort zones even to read about: If anyone reading is easily squicked by sexuality talk, they may want to leave the thread now.

I'm asexual. I've had sex, and experienced orgasms (anhedonically, though I'm not anhedonic in general), but I have little to no interest in either. However, I don't object to sex on principle - it's about as emotionally relevant as any other social interaction, which can range from very welcome to very unwelcome depending on the circumstances and the individual(s) with whom I'm socializing*. Sex tends to fall on the 'less welcome' end of that scale because of how other people react to it - I'm aware that others get emotionally entangled by it, and that's annoying to deal with, and potentially painful for them, when I don't react the same way - but if that weren't an issue, 'let's have sex' would get about the same range of reactions from me as 'let's go to the movies' - generally in the range of 'sure, why not?' to 'nope, sorry, what I'm doing now is more interesting', or 'no, thanks' if I'm being asked by someone I prefer not to spend time with.

Now, I don't generally talk about this next bit at all, because it tends to freak people out (even though I'm female and fairly pacifistic and strongly support peoples' right to choose what to do with their bodies in general, and my cluelessness on the matter is unlikely to ever have any effect on anything), but until recently - until I read that explanation by Eliezer, actually - it made no sense to me why someone would consider being raped more traumatic than being kidnapped and forced to watch a really crappy movie with a painfully loud audio track. (Disregarding any injuries, STDs, loss of social status, and chance of pregnancy, of course.) Yeah, being forced to do something against your will is bad, but rape seems to be pretty universally considered one of the worst things that can happen to someone short of being murdered. People even consider rape that bad when the raped person was unconscious and didn't actually experience it!

According to Eliezer - and this makes sense of years' worth of data I gathered while trying to figure this out on my own - this seemingly irrational reaction is because people in our society tend to have what he calls 'sexual selves'. As you may have picked up from the above text, I don't appear to have a 'sexual self' at all, so I'm rather fuzzy on this part, but what he seems to be describing is the special category that people put 'how I am about sex' information into, and most people consider the existence and contents of that category to be an incredibly important part of their selves**. The movie metaphor could be extended to show some parallels in this way, but in the interests of showing a plausible emotional response that's at least close to the same ballpark of intensity, I'll switch to a food metaphor: Vegans, in particular, have a reputation for considering their veganism a fundamental part of their selves, and would theoretically be likely to consider their 'food selves' to have been violated if they discovered that someone had hidden an animal product in something that they ate - even if the animal product would have been discarded otherwise, resulting in no difference in the amount of harm done to any animal. (I know exactly one vegan, and he's one of the least mentally stable people I know in general, so this isn't strong evidence, but the situation I described is the only one other than complete mental breakdown in which I'd predict that that otherwise strict pacifist might become violent.) Even omnivores tend to have a 'food self' in our society - I know few people who wouldn't be disconcerted to discover that they'd eaten rat meat, or insects, or human flesh.***

The rules that we set for ourselves, that define our 'food selves', 'sexual selves', 'movie-watching selves', etc., are what Eliezer was talking about when he mentioned 'boundaries of consent' (which is a specific example of one of those rules). They describe not just what we consider acceptable or unacceptable to do or have done to us, but more fundamentally what we consider related to a specific aspect of our selves. For example, while a google search informs me that this may not be an accurate piece of trivia, I've never heard anyone claim that it's implausible that people in Victorian England considered ankles sexual, even though we don't now. Another example that I vaguely remember reading about, in a different area, is that some cultures considered food that'd been handled by a menstruating woman to be 'impure' and unfit to eat - again, something we don't care about. Sometimes, these rules serve a particular purpose - I've heard the theory that the Kosher prohibition on eating pork was perhaps started because pork was noticed as a disease vector, for example - but the problems that are solved by those rules can sometimes be solved in other ways (in the given example, better meat-processing and cooking technology, I assume), making the rule superfluous and subject to change as the society evolves. It's obvious from my own personal situation that it's also possible - though Eliezer never claimed that this was the case for 3WC - for certain 'selves' that our society considers universal not to develop at all. (Possibly interesting example for this group: Spiritual/religious self.)

Eliezer didn't share with us the details of how the 3WC society solved the relevant underlying problems and allowed the boundaries of sexuality and consent to move so dramatically, but he did indicate that he's aware that those boundaries exist and currently solve certain problems, and that he needed to consider those issues in order to create a plausible alternative way for a society to approach the issue. I don't see any reason to believe that he didn't actually do so.

* I am, notably, less welcoming of being touched in general than most people, but this is not especially true of sex.

** I find this bizarre.

*** I have a toothache. The prescription pain meds I took just kicked in. If the rest of this post is less insightful than the earlier part, or I fail to tie them together properly, it's because I'm slightly out of my head. This may be an ongoing problem until Tuesday or Wednesday.