Studies that have compared the levels of monogamy find that bisexuals are the least likely group to report monogamy in their relationships. Lesbians are the most like to report sexual fidelity, followed by heterosexuals, and then gay males, and last, bisexuals. However, as a group, bisexuals, both men and women, also appear to be the ones most likely to explore negotiated or ethical nonmonogamy, such as polyamory or other forms of open relationships. As a result, bisexuality offers an interesting window into the ways in which couples negotiate complex aspects of trust, jealousy and commitment.
During the ‘80s, and the Oprah-driven panic of the "down-low" phenomena, where straight-acting men were having often unprotected homosexual sex, and possibly exposing their unsuspecting wives and girlfriends to STD and HIV, there were compelling studies of the stigma of bisexuality. Researcher Greg Herek found that on a spectrum of trust and stigma, bisexuals rank below intravenous drug-users. In fact, lawyers are often regarded as more trustworthy than bisexuals (ouch).
A significant challenge in any of this research is defining the concept of bisexuality. I've treated people who have never had sex with someone of the same sex, and are in monogamous heterosexual marriages. But during masturbation and sex with their spouse, these people could only experience arousal and orgasm in response to a fantasy of same sex activity. Are they straight, based upon their behavior, homosexual based upon their arousal pattern, or bisexual based upon both? Kinsey's original definition was based solely upon reported behavior, and suggested most folks ended up somewhere edging towards the middle (slightly bisexual, in one way or another). The late Fritz Klein expanded this concept, to include a grid, of arousal, behavior, fantasy, and social intimacy, recognizing that our definition of sexuality was inherently limiting.
Research by Nevada's Meredith Chivers suggests that all women may in fact have some element of bisexual arousal, and tendency to react erotically towards same-sex stimuli. There is controversy in similar research with men, as to whether or not a male pattern of bisexual arousal is detectable, though there are certainly many men who identify as bisexual. And research by Lisa Diamond explores the concept of sexual fluidity, evident in women who fluidly move back and forth between different categories of sexuality, without necessarily adopting a sexual label.
Is it different then, when sexual infidelity occurs, but is bisexual in nature? That is, when a committed partner has an affair with a same-sex person? Anecdotally, I can say that over the past decade or so, I've seen increasing numbers of couples where this occurred, and was generally accepted within the couple, as part of their rules of nonmonogamy. "He can have sex with another guy, but he's not allowed to even look at another woman," one wife told me, describing her bisexual husband. Two men in a committed homosexual relationship likewise told me about the one partner's freedom to have sex with other women, but not men.
Confer and Cloud, from UT Austin, the home of David Buss, and evolutionary research on jealousy, recently published research suggesting that men and women respond to bisexual infidelity differently. Men are far more likely (60 percent) to stay with a female partner, after she has an affair with another woman, than are women likely to stay with their male partner, who has sex with another man (only 26 percent of women would stay in relationship). In contrast, 33 percent of women would stay with a man who had a heterosexual affair, and 25 percent of men would stay with a woman after heterosexual infidelity. The authors suggest that this reflect the role of evolution, and the male fear of cuckoldry driving the great disparity between male reaction to heterosexual or bisexual infidelity. They further suggest that female fear of the loss of intimacy and support, also allegedly driven by evolutionary influences, explains the female pattern, that in male partners, "homosexual affairs are more reflective of ensuing abandonment as they evince a more complete absence of emotional intimacy and satisfaction with one's partner."
I'm not sure the evolutionary argument is a very robust explanation here, in explaining the reaction towards male bisexuality, compared to the influence of stronger social stigma towards male bisexuality. Male bisexuals are seen as more disturbed, untrustworthy and dangerous than are female bisexuals. This perspective may have something to do with evolutionary factors, but if so, it seems pretty indirect. Likewise, are the men who accept their female's partner dalliance with another woman really less concerned because of the absence of the risk of pregnancy, or are they just hoping they now have a shot at a threesome with the other woman, if they don't overreact?
A colleague, who has lived in a homosexual polyamorous relationship for almost three decades, has suggested to me that bisexual satellite relationships are less threatening to a same-sex lover, than choosing an external partner who is the same sex as one's primary mate. It "invites less feeling of comparison, both internal and external." A woman in a polyamorous relationship with her bisexual husband told me, "When my husband chooses an external lover of a different sex than me, I don't feel as triggered to compete. I don't feel like I'm at threat of being replaced, that there's something I'm not giving him, the way I might feel if he chose another woman, then I start worrying that she is better than me somehow."
Ultimately, I'm not sure there is a difference between bisexual and heterosexual fidelity, when it comes to what a couple actually does. For couples and individuals to deal with infidelity, it takes communication, self-knowledge, respect and work. These are the same things it takes for any relationship to work. Bisexuals may come to a relationship with certain predispositions or tendencies away from sexual monogamy, but, like all of us, they make choices, and must deal with the consequences of these choices, within their relationships.