So that was intended for a joke, but ok, let's think. Some possibilities:
50 Shades of Rationality - keep the sex scenes but upgrade the characters, HPMoR-style. I haven't read them, but presumably there's some excuse for a plot? So, make villains and heroes both awesomer. This does have the weakness that there's "separation of rationality and erotica" - it's about smart characters, and they have a lot of sex, but it's not necessarily smart sex, so to speak.
A sex scene that the characters have spent (at least) five minutes thinking about and preparing for. Does losing your virginity absolutely have to be a scramble of awkwardness, embarrassment, and rapid ejaculation? Not if the characters expend some effort ensuring otherwise. This is more explicitly rational; it's about how to actually accomplish a goal, the did-you-really-think-about-that-for-five-minutes question. We just make it sexual by setting the goal to "get laid" or "have really good sex".
In BDSM, there seems to be a conflict between the sub not currently being spanked, who wants to be; and the sub being spanked, who (to some extent) wants not to be. Precommitment, discounting, making decisions for yourself-in-the-future, sub-agents within yourself with differing agendas.
Obviously all depends on good writing; but really, anything with a plot - anything longer than five pages and more complicated than "Tab A went into slot B" - should have some room for rationality. The characters have to make some decisions, presumably, and that can be done well or badly.
A sex scene that the characters have spent (at least) five minutes thinking about and preparing for. Does losing your virginity absolutely have to be a scramble of awkwardness, embarrassment, and rapid ejaculation? Not if the characters expend some effort ensuring otherwise. This is more explicitly rational; it's about how to actually accomplish a goal, the did-you-really-think-about-that-for-five-minutes question. We just make it sexual by setting the goal to "get laid" or "have really good sex".
Interesting thing to think about: man...
Last October there was a discussion post on Ada Lovelace Day, and it met with something of a lukewarm reception. Fair enough. There are legitimate criticisms of this particular blogosphere event, and people are welcome to subscribe to those criticisms, or not, as they see fit. Personally, I'm quite fond of Ada Lovelace Day, in no small part because I get a chance to write about one of my nerdy interests in a public place with a reasonable expectation that a lay audience will attempt to engage with it. This year, the occasion falls on October 15th, and as a result I'm currently drafting a short piece on Esther Duflo, a development economist responsible for pioneering randomised controlled trials of policy interventions in developing countries. She's rather prolific, has a shelf full of academic awards, and is a hot tip for a Nobel Memorial Prize over the next few years or so.
So I was thinking about this: I get to talk about the importance of randomised controlled trials in policy-making; I get to talk about evidence-based philanthropy; I get to wrap it up with a don't-put-mustard-on-the-cat closing message of how it's not enough to just care about an issue, you have to be informed on it as well, (and by the way, there's this thing called "effective altruism" you might want to look up); and I can expect a reasonable number of readers to actually engage with it, because it's ostensibly written about the work of an interesting woman on Ada Lovelace Day.
You can probably see where I'm going with this by now.
Whether or not you think it's valuable to publicise the work of women in STEM, it is an excellent opportunity to sneak assorted pro-rationality memes under the radar to an audience that wouldn't necessarily engage with them otherwise. Less Wrong has a lot of eloquent people with knowledge across a wide assortment of different domains. I'm curious as to what we could come up with if we made a concerted effort to do this.
For that matter (and Harry Potter fanfic aside), it's an interesting question as to what other popular internet phenomena can be co-opted for this purpose.