Disclaimer: Twilight and its character belong to S. Meyers, etc. I'm only borrowing.
They can't have Bella.
When Edward leans into her kisses, tongue exploring her mouth, non-beating heart wanting to speed up with the excitement of the moment, he knows he has to stop. Because the natural progression is not just another piece of him exploring her, but for his lips to wander down her jaw, part against her throat. And only the slightest lack of control would bring instincts to the surface that he might not be able to stop.
Jacob could have her without hurting her. Sex isn't anger (or it wouldn't be, with them), so he would stay distinctly non-furry. He might be a little rough, but he thought that she wouldn't dislike it. And as her arousal built, as her temperature tried desperately to come anywhere near his own, his teeth would nip against her throat, mouth sucking marks against the delicate skin that had nothing to do with a passion for her blood.
But Edward keeps him from Bella, even more than Edward keeps himself from Bella. As much as Bella loves Jacob, she'll always want Edward more.
And that means neither of them get her.
It doesn't stop the wanting, the needing, or the inevitable meetings. Because Edward is always covered in Bella's scent, and Jacob's often the same. And after months of exposure, their own scents don't sicken each other. They can ignore them.
Neither has to be careful. When Bella asked what happened to the bed in Edward's room, he just said he felt like having it replaced. The excuse worked all five times, because the Cullens always seemed fickle about material possessions. No one asks Jacob why so much of his clothing ends up shredded rags, they don't think they have to.
Blood loss doesn't hurt a werewolf that much, even in human form. Edward's venom stings like antiseptic on the wound, but does little else. Jacob is hardly the first of his kind to be bitten by a vampire, though he's sure not a single ancestor had the same sort of experience.
Though he dreads it, Edward's eyes don't turn red. Which leads to more caustic comments about Jacob's animal nature. Which Jacob takes advantage of--after all, he wouldn't be the one practicing bestiality.
They may have actually started to get along.
And when they think of meeting again, of their bodies grinding together in a rhythm that might be older than either of their races, they start to forget why they're doing it. Edward's eyes are as deep as the forest and Jacob's smiles are bright like the sun, and they get lost.