Google Search Dropped Traffic To XahLee.org

hi, i have question about how google represent my website, and a sudden vertical drop of traffic from google, represented by this image:

http://trends.google.com/websites?q=xahlee.org

Google Trends on XahLee.org, as of 2009-06-16 .

I have this math project on my website: Visual Dictionary of Special Plane Curves .

This project is the primary content of my website. It has been top search result and traffic for my site from 2000 to 2008 june. (basically, roughly speaking, 70% or more traffic on my website is about math topics in project and other math contents.)

However, i noticed today, that google webmasters sitelink does not even list it. Instead, my website today is represented by keywords such as the following top 10 keywords result:

ass blondie orgy virgin young girl lara croft curves rebel flag girls girls girls parabola lady guy youporn.com sex position tony braxton debbie does dallas

which goes to 2% of my website's pages that are commentary about human sexuality. Google Analytics basically reports the same. (total number of pages on my site is about 3900.)

There seems to be something drastic that happened within the past half year, starting in 2008 June. Basically, the math content, which is the main content of my site, has been downplayed to almost non-existence, while about some 2% of my website's content, that are rather trivial blog style commentaries on human sexuality, became the main representation for my website. My site traffic from google search has been reduced to some 1/3 of 2008 june.

Can anyone give me some idea about what's going on? Here's some specific question i have.

1. why is my major project http://xahlee.org/SpecialPlaneCurves_dir/specialPlaneCurves.html that has been around for over a decade and widely linked by edu institutions, now seems largely delisted.

2. i don't want the 5% of my blog writings on lurid matters such as porn or sexuality, became my site's representation, because 95% of my site's content are on strictly math and computer science topics. How can i do this?

3. Starting maybe around 2006, i noticed a huge amount of machine-generated spam sites or blogs that insert my name Xah Lee, or snippets of my site's writings. What can i do about this? (often, if i just search my name xah lee or my site xahlee.org on blogs, newsgroups, the results are mostly machine generated spam.)

Thanks.

Xah ∑ http://xahlee.org/ ☄