I don`t know how long you guys last, or what boring sex you have.. but i`m definitely out for at least an hour after some good lovemaking :D
You must be doing it wrong!
(and before i get comments on my fitness: as a fire-fighter i`m -de facto- in excellent shape ^^ )
Burning calories through sex is a myth, it is proven that having normal sex(not some all night marathon) burns just as much calories as getting up a staircase.