In Carnal Knowledge Faye Flam discusses the origins of homophobia in feelings of disgust.
Please don't freak out on me, gentle reader, and read the whole of the next paragraph.
In a sense, disgust about homosexuality is just as good a starting point for the ethical condemnation of homosexuality as disgust over the violation of consent is a starting point for the ethical condemnation of rape, or disgust over the murder of children is a starting point for the ethical condemnation of the Iraq war. But that's all this disgust is: A starting point. Throw some actual thought into the matter and the disgust at homosexuality, unlike rape, murder and war, trips on its own feet six inches from the starting point.
The necessary/sufficient distinction seems to confuse a lot of people: While disgust is necessary for ethical condemnation, it is not sufficient. If we were to condemn everything that disgusted someone, we'd condemn everything, leading to a degenerate (pun intended) ethical system. Rape and murder disgust almost everyone; tens or hundreds of millions of people do not find homosexuality at all disgusting.
It's also useful to consider the psychological character of this disgust. Homosexuality disgusts people primarily because they put themselves in the act itself, not in the minds of the people engaging in the act. Rape and murder disgust me not because I don't want to participate in such activities, but because I have empathy for the suffering of the victims. Consensual homosexual sex, however, brings joy and happiness to the participants, and, everything else being equal, I approve of joy and happiness. Disgust over homosexuality requires that someone suspend his empathy, his connection to the feelings of others; I strongly disapprove of the suspension of empathy.
Most importantly, this supposed "disgust" seems to be strongly correlated to mental slavery to primitive superstitions (a.k.a. Abrahamic religious belief). No one needs to reference any scripture to condemn rape, murder, theft, and the like. When I see the primary "intellectual" justification for an ethical belief coming directly out of the prejudices of Iron-age goat-fuckers and medieval child-rapists, I call bullshit.
As Eric Schwartz so succinctly puts it, "Keep your Jesus off my penis... I don't tell you how to pray, so don't you tell me how to screw."