1 Marvin Gaye breaks it down in “Let’s Get It On”—what could be simpler? Psychologists at the University of Texas at Austin take a different view. They surveyed 444 people [pdf] and found 237 reasons why people have sex.
2 “I was drunk” made the top 50 for women; “to get a favor” made the top 50 for men.
3 Believe it or not, biologists cannot agree on the reason for sex either.
4 The standard evolutionary argument is that the mixing of DNA between two members of a species introduces more variation into the gene pool. But asexual reproduction is far more efficient.
5 One leading hypothesis suggests that sex won out over DIY as a defense against parasites, which would be less harmful in a varied gene pool.