To quote Asayogure:

In the course of introducing friends to anime and fanfics the question
has often been asked of me, why are lemon fics called that?  I have
never had an answer.  So now I put the question to you.

I read the FAQ howzabout you?

That's nice, but it doesn't actually answer his question, which is:

Why is a lemon called a lemon?

The answer, I have been told, is that one of the first/famous adult 
manga had the word lemon in its title (cursed memory, can't remember
what it actually was). Anyway the name was adopted to refer to the
sub-genre. That simple really I believe.
-----------
Rarstarr,
David Farr
http://homepages.ihug.co.nz/~djfarr/
djfarr@ihug.co.nz