Rumor
Rumors are nasty things that spread like weeds and are just about as hard to get rid of. Harder, maybe.
So when the rumors started that Kenny was going behind Butters back with Bebe, Butters didn't know what to think. Bebe was prettier and Kenny had made such a show of liking breasts when they were younger, something he lacked.
But when Kenny would come home to the apartment they shared and smiled at him like he was the only person in the world, Butters knew that the rumors were just that: rumors.
He was the one Kenny ever loved.
Bebe is probably in a corner somewhere, wondering why the hell people think she's with Kenny.