I just figured out why Rincewind can never get together with a female. The most important female in his life is The Lady. He's her little pet and she seems extraordinarily fond of him.

Methinks The Lady might be the jealous type. I mean you have to have noticed what astoundingly bad luck Rincewind has hooking up with women. He could fall into a harem full of them and not land on one. The Lady loves her pet, but she's not going to share him with another female.

(As a concession to threats from the militant ninja slashers I'm confining my theory to females. Feel free to continue to hook him up with males and male leaning pineapples.)