Well, maybe I can try to take a crack at it.

1) Less Wrong is already incredibly male slanted. According to the most recent survey I know of, Less Wrong members report themselves 96.4% male. Thus, whatever effort is put into a 'dating thing' will certainly end up wasted.

1a) Addendum: Actually, we're an incredibly narrow group demographically speaking. We could probably make a decent run at a 'Gay White 18-30 Athiest high-IQ dating site'. I'm not personally interested in that, but I'm sure someone could run with it.

2) It's been noted before that we have a gender imbalance, and that such an imbalance harms the art of rationality as a whole. Such behavior on a Less Wrong partnered endeavor could easily push away women here and prevent new women from joining. "...my goal [are to] create rationalists not create female rationalists. But if half of the audience is being filtered for some silly avoidable reason, then I want to fix that." And this qualifies as a silly, avoidable reason.

3) Part of Less Wrong's draw is that the members generally appear high status and rational; those who join can become high status and rational too. Advertising our singles (all loaded on one gender) will bring down our status as a group. Advertising that our members are trying to catch fish in the desert (96.4% male!) will harm the perception of rationality even further.

4) By allowing such an option into hypothesis space we'd push people away from Couchsurfing. A potentially usefully tool would lie underutilized because people would have to worry about being sexually assaulted by strangers on the internet.

5) Even if everything went exactly as 'hoped' and an attractive girl actually took you up on the offer, it still comes out squicky. You have a girl, isolated in a stranger's house in a strange city, with nowhere to go and (since they're already couchsurfing) no money, who has already consented to sex as payment for shelter. That's not exactly rape, but it doesn't offer very much in the way of withdrawing consent. It could easily go bad; hell it sounds like a recipe for making badness.

In summation:

1) Won't happen, 2) lets not push away women more, 3) makes us look bad, 4) stops people from couchsurfing, 5) WTF?