I saw this on /r/deadbedrooms earlier.

http://www.theguardian.com/science/2010/jun/18/love-sex-masturbation

...the hormone Prolactin makes people not want sex. Its essential, otherwise humans would only want sex and not even take time to eat. It spikes after sex... and it rises after relationships get older. At 1 year, then at 3 years it rises again. (Sounds like the typical stages of a relationship doesn't it?) It also rises more in women than in men, and although they cant put humans in cages, they found that monkeys they tested only had high prolactin levels after sex or... when they were brought in and put into cages, and they realized they were trapped. To me it explains why so many women have low libidos and then when getting out of a marriage, suddenly turn into fireballs of sexual desire: they aren't trapped. Prolactin is apparently what pushes us to want new partners.

For reference:

/r/TheRedPill/comments/22t9ls/study_a_womans_sex_drive_begins_to_plummet_once/

http://readability.com/m?url=http%3A%2F%2Fnews.bbc.co.uk%2F2%2Fhi%2F4790313.stm

http://i.imgur.com/Zreanes.png

All things combined, this may very well explain the phenomenom of the "honeymoon phase" ending at the 1 year mark, and why women's libido crashes as the relationship wears on.