Hre's an interesting question - what lemon would be so 
utterly weird and wrong that you'd run screaming?  There've
been a lot of different pairings out there!

My nightmare:  an orgy with the following:

Azusa Shiratori (Ranma 1/2)
Yagami Ibuki (Maison Ikkoku)
Ran (UY)
C-Ko (Project A-Ko)
Leon McNichol (Bubblegum Crisis)

Can anyone top this?

Shampoo and Cologne... Hmm. No, she'd actually deserve that.

Umm... How about ... Happousai and Tsubasa?

Hmmmmmmm...

Cutey Honey and the Overfiend Better yet, Shampoo and Chojin.
Ack, you wanted *nightmare*... Chojin and Ukyou?

Or that monster thing from Flair and the whole cast of Ranma 1/2?

Kun-chan...