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2.
Having sex more than 3 times in one night. Each time lasting more than an hour.
Last night me and my boyfriend had marathon sex, we didn't go to sleep until 6 this morning!
by Hannahboo32 October 14, 2007
 
1.
Sex that continues for hours per session, punctuated by winded intermissions wherein the parties involved sweatily seek to restore the normal oxygen-carbon dioxide mix in their blood; Sex that burns as many calories as a marathon.
Our marathon sex session came to an end when both of us were simply too tired to continue.
by howe March 24, 2005