Maybe he has plans for an elaborate swing.
It's hard to capture the thrill of being caught in the bedroom, unless you leave the door open.
Weather. How else can you have sex while standing in the middle of a thunder storm? Or blizzard? (My testicles shiver at the thought.)
Maybe he has something messy in mind, a food or a mud fantasy, that he doesn't want to bring indoors.
That's all I can think of. I'm sure there's more.