Because you can't have bone-shattering orgasms all the time. If you did, you'd never leave the motel room, things wouldn't get hunted, people wouldn't get saved, and the family business would go to hell.
And because you can't always have "I've wanted you for so many years" sex. "Love you so much" sex. "Only you only ever been you" sex. Because then the Man Police would kick down the door and rip the Man Cards out of your wallets.
Sometimes, you just gotta fuck.
Jeans pulled half-off in a truck stop bathroom, pounding into Sam without finesse, watching yourself in the polished metal mirror, jerking Sam's cock until he comes, red-faced and panting, all over the rust-stained sink. Frenetic handjobs inside the Impala, grinning at each other, seeing who can make the other get off first. Sam rubbing himself off against the crack of your ass in the shower and then turning around under the warm spray for you to do the same. Sinking to your knees in an alley without warning, unzipping Sam's Fed Suit slacks, ignoring his gasp of shock, and working your mouth and tongue on him hard until he comes for you. Too tired for the whole elaborate production, just a simple sixty-nine, lying on your sides on yet another motel bed, sucking until the bitter-delicious liquid fills your mouths, and then passing a whiskey bottle back and forth until sleep comes.
Sometimes Sam tries to see how fast he can make you come. The faster you shoot into his mouth/hand/ass, the more proud of himself he is, beaming at you like he just aced a test.
Sometimes you fall into bed, exhausted, with no more than a quick kiss, and when you wake in the middle of the night, you just…take him. Lube your half-hard cock up quietly, slot in behind Sam, and work it in quickly, get hard completely once you're all the way in, opening him up for you with your cock. Sam wakes up shuddering when you do this, loses his fucking mind, to wake up to you already inside him, fucking him, taking him. He comes so fast like that. You both do.
Yeah, the hearts and flowers sex is awesome. The kinky, set-out-the-implements-in-advance productions are mindblowing. But sometimes? Sometimes, you just gotta fuck.