I've been a Wikipedia article for several years, and am the primary author of several highly-rated articles, so I'm pretty familiar with Wikipedia and how it works. And I'd add that the idea of a sex-positive wiki isn't appropriate for general Wikipedia for a reason that you haven't mentioned – the principle of Neutral Point of View, or NPOV for short. By its nature, Wikipedia articles are not supposed to take a stand that's pro-sex-positive or anti-sex-positive, pro-abstinence or anti-abstinence. If there is any kind of controversy or disagreement about a particular issue (like porn or prostitution), all sides of the issue are to be given equal weight. Wikipedia articles are also bound by the principle of "No Original Research", which means that that the editors own analysis and arguments are not allowed in the article – only arguments that are already found in citable, published sources can be given there. If you have an issue with a particular point of view given in an article and think it needs to be balance, it can only be balanced by reference to an already-published criticism, not your personal point of view.

A lot of people don't like this and avoid contributing to Wikipedia for this reason, but on the other hand, I think the idea, that it least in theory, if not always in practice, Wikipedia isn't supposed to be slanted toward any kind of partisan view to be one of its strengths. And, personally, I consider writing within those restrictions to be an interesting challenge, and one that really forces you to learn about points of view you don't necessarily agree with, and a lot about separating fact from opinion.

Nonetheless, I do think there's a lot of abuse on Wikipedia and lot of outright "point of view pushing" there, sometimes thinly concealed, sometimes not. And it seems like articles about sexuality issues often have particular problems with this. I'm actually involved in one of the more brutal disputes around an article, namely the Wikipedia article on Melissa Farley. You can read the talk page for the whole thing, but basically some prostitution abolitionist is attacking the article as "biased" because it happens to include information on the criticisms that have been made of Farley's research methods and views, and info on Farley's background as an anti-porn activist. (And, yes, the very use of the term "sex work" in this article came under attack as well.)Another problem article is an odd little article called "Adolescent Sexuality in the United States" which is packed with a disproportionate amount of dodgy pro-abstinence propaganda largely contributed by one problem editor. I've also found from these disputes how inadequate Wikipedia is in practice for putting the brakes on abusive editors.

But anyway, I think starting a sex-poz wiki is an excellent idea – POV wikis definitely have their place – there is already an anarchist wiki and a conservative one, for example. I'd love to contribute to a sex-poz one.

(Oh, and another aside, you mention the Sexology and Sexuality WikiProject – I also wanted to point out the Sex Work Task Force within that project that just got started. Some readers here might be interested.)