Because it's been bugging me a while now

Why does the word 'lemon' denote a pornographic fanfic?

I mean, why 'lemon'? What is there about citrus fruit that is reminiscent of sex?

I mean, on the one hand you have this little yellow dimpled roughly oval thing, on the other hand you have...well, you know.

If lemons are famed for anything, I thought it was staying put. I mean, when I was a kid and in the way or not doing something that I should of been then my mother would say 'Don't just sit/stand there like a lemon!' I can't really see the relevance, myself.