I think you're reading too much into it--it would be a big change in the gender ratio, not necessarily anything else. Personally, I don't think it'd matter all that much otherwise, as far as the discourse here is concerned.

why it is likely that people who want to join have that skewed sex ratio?

People don't find sites like LW out of the blue; they need to find links from some other site, have some reason for doing a web search that leads to this site, or have it mentioned by a friend.

• If memory serves me, statistically, most friendships are same-sex, so that vector will have a similar sex ratio.
• LW is a small site on esoteric topics, therefore the chance of finding it from a random Google search are small, so that vector is likely to have little impact.
• Most of the other sites that would currently be inclined to link to LW are topically related to computer science, philosophy, economics, science, athiesm, technology, science fiction, &c.--most of which are interests that also have skewed gender ratios to some degree, some more or less so than LW[1].

That is also my answer to "why I don't expect it to change", at least not without deliberate outreach of some sort to compatible communities with less gender skew.

Short version: It's a self-perpetuating situation for various reasons.

[1]: I've been in at least one technology-oriented community where male-to-female transsexuals outnumbered biological females among active participants. I don't think I need to point out how statistically unlikely that is.