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.

When you're being horny (especially around friends) and as a result shame ensues.

Having sex more than 3 times in one night. Each time lasting more than an hour.

Sex that lasts over an hour, with continued intent; if only an hour and a half or so, partners may only be distracted during sex. During 'marathon sex,' as opposed to marathons, small breaks are sometimes taken, akin to stretches during exercise, which is part of what the 'marathon' is... exercise, for both partners, in every case, though! For it to last so long, mutual exercising, and most especially pleasure, throughout the entire event, are both prime requisites.