In a message dated 3/23/99 5:19:37 PM Eastern Standard Time, djfarr@ihug.co.nz
writes:

 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.
 

Long before I got into fanfiction, I remember that the first porn anime I ever
saw out was called Cream Lemon. Maybe that's it.

James