As I was looking for something else, I found Pepper Mint's paper "Bisexuals and the Slut Myth" that was presented at the 9th International Conference on Bisexuality, 2006:

... First ... Why is the connection between bisexuality and sluttiness so persistent, to the point of ignoring the reality of large numbers of [happily monogamous] bisexuals? ...

Second, rates of nonmonogamy seem to be relatively high among bisexuals.... I personally have observed this anecdotally in the polyamory community. Fill a room with poly people, and the number of bi people in the room ranges up to sixty percent.... What gives?

Pepper Mint proceeds to deconstruct the bisexual-as-slut myth as arising from two false assumptions:

... the “one of each” myth.... is the common idea that bisexuals will not be happy unless they are dating one of each, namely a man and a woman....

The problematic assumption here can be stated simply: men and women are so different that desire for one is an entirely different beast from desire for the other. This assumption is sexist, as it is based in the false assumption that men and women are entirely different. It is also a defining feature of heterosexism....

There is a culture-wide assumption that verbalizing a sexual desire inevitably leads to attempts to satisfy that desire. I call this the immediacy of desire.... One of the primary functions of the immediacy of desire is as an enforcement tool for monogamy.... Preventing the expression of desire removes it from the arena of discourse, and also delegitimizes it....

We now have two false assumptions that together form the basis for the “one of each” myth. First, it is assumed that bisexual desire is actually formed of two different and distinct desires. Second, it is assumed that each desire must be satisfied (within a relatively short timeframe) for the bisexual person to be happy. Doing so would constitute nonmonogamy.

He goes on to discuss the invisibility of monogamous bisexuals, and the implications of this slut myth on bisexual and nonmonogamy activism.

(Emphases added.
 Read the full paper.)