Browsing the research databases, this study certainly caught my eye. "Why humans have sex" is written by Cindy Meston and David Buss and published in the August edition of Archives of Sexual Behavior.
The study's importance lies in understanding the cognitions and emotions underlying the behaviour and it is an area that has received relatively little attention. Reasons for engaging in sexual behaviour had been thought to be relatively few, mainly related to the desire to reproduce, experience pleasure and relieve sexual tension. However, the 444 participants in this study managed to come up with a heady 237 reasons, including those anticipated but also reflecting a whole range of psychological motivations, including spiritual ("I wanted to feel closer to God", reason 7), vengeful ("I wanted to hurt/humiliate my partner", reason 78), a sense of obligation ("I felt like it was my duty", reason 48) and coercion ("The person demanded I have sex with him/her") among others.
Some of the more unusual reasons include "The person was a good dancer (Reason 201)" and "The person was famous and I wanted to be able to say I had sex with him/her (Reason 23)".
Good enough reason to stop Practical Man watching 'Dancing with the Stars' I'd say.
On a more serious note, the study revealed four major factors underlying a decision to have sex: physical reasons (stress reduction, pleasure, physical desirability and experience seeking; goal attainment (resources, social status, revenge, and utilitarian); emotional (love, commitment, expression); insecurity (self-esteem boost, duty/pressure, mate guarding).
When, in a further study, respondents were asked to rate the importance of each factor in respect of their own experiences, significant differences were found according to gender (which probably comes as no surprise to most of us) and personality type.
If you'd like to read more about this study or the 237 reasons, the link above provides full access to a text version of the journal or you can source it via the reference provided below. Meanwhile I'm off to conduct a counter study. "Reasons why humans don't have sex."
Meston, C. C., & Buss, D. M. (2007). Why humans have sex. Archives of Sexual Behavior, 36, 477-507.