"Originally, I thought that we exhaustively compiled the list, but now I found that there should be some added..." wrote Cindy Meston, co-author of "Why Humans Have Sex," in the Archives of Sexual Behavior (Volume 36, Number 4, August 2007). Denise Duhamel and Sandy McIntosh have done just that in this delightful compendium that adds 237 more reasons. It's an exhaustive list, but it still doesn't exhaust all the possibilities. So be warned, you'll want to find some more.