When Michael Sam kissed his boyfriend on television to celebrate his successful draft into the NFL, some viewers were disgusted. The charged comments that followed demonstrated that when it comes to public displays of gay affection, some people have a gut reaction to recoil. But why?
The answer to that question is not fully known, but scientists are beginning to establish an understanding of how biology and the environment may interact to form such reactions.
First, it's important to understand that disgust in humans can be good. We should recoil from the truly gross things that can harm us—festering wounds, rancid meat, and feces, to name a few examples, are dangerous incubators of infection. "Disgust is a part of what is referred to as the behavioral immune system, which protects us from dealing with items and individuals that might make us sick, that might kill us," says Patrick Stewart, a political scientist at the University of Arkansas (and not of X-Men, Star Trek fame).
But it's not adaptive when healthy groups of people are reacted to in the exact same way as if they were pathogens.
Here's the state of the science of disgust right now. Conservatives are thought to have a greater propensity to be disgusted than liberals do. Many studies corroborate this idea (see here, here, and here). One illustrative study in 2011 found that people who were more physiologically disgusted, a reaction that was measured by skin reactions, by this photo of a man eating a mouthful of worms were also more likely to self-identify as conservative. They were also more likely to have a negative response to gay marriage.