Why do some people feel threatened by other peoples’ sexual orientations?
There are two primary reasons why some people are threatened by others’ sexual variation: religious/moral judgment that it is wrong to have sex for fun, and fear. Fear of difference, fear of their own desires, fear of the unknown.
First, moral disdain. Numerous religious prohibitions caution that sex should only be for making babies, and that having sex for fun is indecent (even though pretty much everybody does it because it can be a lot of fun). This prohibition implies that recreational sex is somehow wrong, morally appalling. Discovering that recreational sex can be enjoyed in an honest and thus ethically sound manner can be revolutionary for some people, especially those who had previously thought that the only two choices were monogamy or cheating. Lying actually is ethically wrong, but having multiple partners is not unscrupulous if everyone has negotiated and consented to it.
The second reason that people are uncomfortable with others’ sexualities is fear. In many important ways, humans are “hardwired” psychologically and socially to prefer people who are like themselves over people who are different from them. People who are heterosexual, monogamous, and vanilla (the word kinky people use for people with conventional sexual proclivities) may feel fear and discomfort with gay, non-monogamous, or kinky people simply because they are different. Others who may have inklings of those desires but have not admitted it to themselves will often fear that sexual “other” because of how they stir those uncomfortable desires.
In regards to polyamorous people, this comes out most clearly in what I call the polyamorous possibility, or becoming aware of the potential to openly love multiple people at the same time. Realizing polyamory as an ethical relationship option can be a significant shift for some people: Once it has occurred to someone that honest, openly conducted multiple partner relationships are possible and can be managed in an ethical manner, they can never unthink that idea. They have become aware of the polyamorous possibility and, regardless of whether they consider polyamory themselves or simply reject it out of hand, they can never again be unaware of consensual nonmonogamy as an option.
The polyamorous possibility can seem even more threatening than simple gayness. While not everyone experiences same-sex desire, most people who have been in a sexually-exclusive relationship have had the experience of being attracted to someone else besides their partner. That means that almost anyone has the capacity to be polyamorous in a way that not almost everyone could be gay, simply because not everyone experiences same-sex desire. Polyamory, and its slutty sister “swinging,” can be especially threatening precisely because so many people can easily relate to the desire for new sexual partners.