And now we have a logical explanation, courtesy of a RadarOnline source:
It's a business, man.
As in, dating the world's largest object of tween affection is good for her career, even if it stunts her libido and makes her miserable. So if your Selena-boner goes limp every time you picture her in bed with the Biebs, remember that their romance is a total sham. Think of it like one of Michael Jackson's marriages.