Once they came up with that long list, Meston and Buss asked 1,549 college students taking psychology classes to rank the reasons on a one-to-five scale on how they applied to their experiences.

"None of the gender differences are all that great," Meston said. "Men were more likely to be opportunistic towards having sex, so if sex were there and available they would jump on it, somewhat more so than women. Women were more likely to have sex because they felt they needed to please their partner."

But this is among college students, when Meston conceded "hormones run rampant." She predicted huge differences when older groups of people are studied.

Since her study came out Tuesday, people are coming up with new reasons to have sex.

"Originally, I thought that we exhaustively compiled the list, but now I found that there should be some added," Meston said.