Got your point, Matt. Being gay is not a disability, but differences can be unsettling for some. What we don't know we don't understand. What we don't understand we tend to fear, and what we fear we usually dislike because that's easier than trying to understand.
My opinion is that it's the person's own comfort level with faced with something "different" that fuels the nervous "some of my best friends are..." reaction. Sometimes I've felt as though they were trying to over-compensate in an effort to be polite, too, and sometimes it's a flat-out ruse to blanket their distaste.