Maybe this will help a little:
http://www.pripix.com/features/ghost.htm
Apparently, Yaoi-sensei is a professor of paranormal studies.
He also happens to have a somewhat unfortunate name.
Yes,
yaoi is the term for gay (as in male on male) works of manga (especially doujinshi), anime, or whatever. These can range for romance-novel type things which are more melodramatic and focused on the romance/love angle, to the "soft-core" works that may be a little further down the road of explicit-ness, to fairly hardcore stuff involving rape and whatnot. Many Japanese girls (and at least 1 Chinese girl I know personally) LOVE this stuff. Most of the fan works involve male protagonists from games, comics, & anime expressing their, er... "true feelings" for one another. Popular subjects include the Gundam Wing boys, any guy from any Final Fantasy, the male cast of Harry Potter, the boys from Yu-Gi-Oh, and the flavour of the month in the USA right now, Naruto.
I tend to prefer
yuri, which is the lesbian (female on female) type stuff, that has the same range as mentioned above, from mushy to, well, let me put it this way -- there's a manga called Super Fist Ayumi. Get the picture?
So yes, I admit I have a nice little collection of Sailor Moon and Evangelion manga where the girls...
like each other.
A lot. 
Ask me sometime about Digimon and Pokemon doujin... heh.
Hentai can mean lots of things, and is abbreviated "
ecchi", or the Japanese pronounciation of "H".
To call a person
hentai, you could mean "pervert", or just plain "weirdo".
And thus ends today's lesson.
Hah!