Here's an interesting chart from an article by a research psychologist who's studying the correlation between homophobia and disgust. He lists few established predictors of homophobia like sexism, religiosity, conservative politics, sexual anxiety, and impulsiveness (!), and goes on to describe his own work on correlations between disgust and homophobia.
The chart below is based on some of his findings - people with high sensitivity to disgust (like they're easily upset by bugs or poop or mold) are on the right, low sensitivity is on the left. They were divided up and given a gay story or a straight story to read, and the resulting level of disgust was recorded.
In other words, the argument is that the same psychological response to bugs is what some people literally feel in response to male homosexuality. This gives a whole 'nother dimension to "Gays are icky."
Here's how he describes his work:
There is a great deal of evidence that linking sexual prejudice and sexual minorities to disgust. One way of studying this is to use a measure of disgust sensitivity. This is way of measuring one's emotional disposition for feeling disgusted. The most popular scale was created by Jon Haidt (you can check out the scale at his website). In the studies I'm doing right now, I use the Three-Domain Disgust Scale, which measures sensitivity to sexual disgust, pathogen disgust, and moral disgust. Another researcher has found that sexual disgust sensitivity (measured with the Haidt scale)--but not trait anger (one's emotional disposition for feeling anger)--predicts anti-gay male prejudice, with higher sexual disgust sensitivity scores corresponding with great anti-gay male prejudice (Tapias, Glasner, Keltner, Vasquez, & Wickens, 2007). And consistent with the idea that prejudice toward different groups is associated with different emotions, they also found that trait anger--but not disgust sensitivity--predicted anti-Black prejudice. I have replicated this in my own (as yet unpublished) work, finding that sexual disgust sensitivity and pathogen disgust sensitivity (physically disgust things like stepping in dog poop) predicts both anti-gay male and anti-lesbian prejudice.
Based on these findings, I conducted an experiment this spring looking at emotional responses to sexual transgressions. My participants read a story about a male employee who seduces his boss in a failed attempt to get a job. In half of the stories, the boss was male (gay condition) and in half of the stories, the boss was female (straight condition). I measured their emotional responses to the story and they also completed the disgust sensitivity and sexual prejudice surveys. Those who read the gay story were more disgusted than those who read the straight story, but there were no differences in anger. When I looked to see if disgust sensitivity played a role, I discovered that those who were high in pathogen disgust sensitivity were more disgust by the gay story than the straight story, but there were no differences for those who were low in disgust sensitivity.
And this explains Peter Labarbera's life's work:
There is research which suggests that when disgust becomes attached to something, it is there permanently. It's almost like a drop of black paint in a bucket of white paint. You can't get rid of it completely. If disgust is attached to sexual minorities, I am interested in the implications when LGBT persons are being evaluated in a domain that is unrelated to their sexuality.
Another group of psychologists are working on the bigger picture of our disgust response, and their research shows that it's about more than just a physical response to real sources of pathogens (like bugs or bad food):
According to the principle of preadaptation, a system that evolves for one purpose is later used for another purpose. From this viewpoint, disgust originates in the mammalian bitter taste rejection system, which directly activates a disgust output system. This primal route (e.g., bitter and some other tastes) evokes only the output program, without a disgust evaluation phase. During human evolution, the disgust output system was harnessed to a disgust evaluation system that responded not to simple sensory inputs (such as bitter tastes) but to more cognitively elaborated appraisals (e.g., a cockroach). Initially, the evaluation system was a food rejection system that rejected potential foods on the basis of their nature or perceived origin. This was the first "true disgust," because it engaged this evaluation system. Later, through some combination of biological and cultural evolution, the eliciting category was enlarged to include reminders of our animal nature, as well as some people or social groups (2). This process had adaptive value, because by making things or thoughts disgusting a culture could communicate their negativity and cause withdrawal from them.[...]
According to a possible three-layer scheme of disgust analysis, there are three pathways through which an elicitor could activate the disgust output program (see the figure). The core route elicits a set of disgust evaluations (appraisals, feelings, and contamination cognitions), which in turn lead to the disgust output. What about routes involving moral violations of incest and unfairness? It may be that incest and other corporeal (divinity) violations activate the disgust evaluation system, just as do elicitors of core disgust. If unfairness and other moral violations that have no corporeal element trigger the disgust evaluation system, then they represent the furthest expansion of the "oral to moral" evolution of disgust.
The issue with homosexuality is that there is a physical element that raises disgust in some people - anal sex. But even if they manage not to think about that, they still prefer a more abstract form of purity over impurity - what's described here as "divinity." But the psychologists stress that this isn't an abstract reaction, but a real, physical disgust triggered by abstract input:
Alternative views by a number of scholars propose that the link between morality and disgust is largely a metaphor (5), construed as such because it bypasses the disgust evaluation system. But the link is not "just" a metaphor. Unfairness and other moral violations may directly affect the disgust output system, after processing by some other evaluation system, or these violations might simply activate the verbal label "disgust," which would then activate the disgust output system. The outcome of either route would include the facial expression of disgust. The Chapman et al. observations are consistent with both these alternative routes as well as the one that uses the disgust evaluation system. But only if evidence is found for a route from unfairness to the disgust evaluation system can it be concluded that disgust at unfairness is "the same" as disgust that is elicited through the core route (such as in response to cockroaches).
There is evidence that violations of the ethics of divinity (especially violations of food and sex taboos) engage the full disgust evaluation output. People feel disgust for divinity violations (3). There is also a link between incest and oral inhibition (such as nausea, gagging, and loss of appetite) (6). And contaminating cognitions accompany divinity violations (7).
It explains why homophobes try to link gays to sodomy, STD's, "plumbing," feces, and children (the purest people around who need to be protected from all this dirtiness). They can't avoid it themselves - they've made the link between homosexuality and disgust and can't help but feel it every time the topic is brought up.
It also explains why gay activists try to do the opposite and clean up our collective image for ballot initiatives, going so far as to scrub the gay away. They're trying to avoid a psychosomatic reaction to homosexuality itself that can't be eliminated in a short campaign season.
And then there's the link between disgust and internalized homophobia that goes deeper than religious condemnation or literal interpretations of heaven and hell (especially evidenced by non-religious, progressive queers who have to get over self-hatred over their sexuality anyway). I'm sure this is the sort of thing that's hard to detect oneself, but it does explain a lot if we're actually, physically disgusted by our own selves.