A mashup of the old animated Star Trek series and William Shatner's cover of Pulp's "Common People." Done in the style of K/S stories, kinda. Therefore, a "slashup." For those complaining, Joe Jackson is the singer in this duet, but Ben Folds produced the entire album it comes from, which I think is where the confusion is coming from.

On Kirk/Spock stories, from Wikipedia:

Almost from the beginning, fans noticed the loving nature of the relationship. A few fan writers started speculating about the possibility of a sexual relationship between Kirk and Spock. The Kirk/Spock phenomenon eventually took on a life of its own, and became one of the driving forces in Star Trek fanzines during their heyday.