"Gaydar" obviously only picks up discernable patterns, though -- it isn't accurate in discerning whether ANYONE is gay, it's only "accurate" in discerning those who are *discernably* gay to a higher precision than those without "gaydar" senses can pick up.
I don't think anyone disputes that there are gay social stereotypes that some people do exhibit (and that's okay), but I think what Alceste is (correctly) saying is that it's not indicative of how all homosexuals and bisexuals are.
You wouldn't be able to tell I'm a lesbian just from interacting with me, for instance, if I didn't say anything about it.
All makes perfect sense. Small addition on your last point though...
There was a gay guy working at the same factory as me whilst I was working my way through university. He was pretty blokey, and also realised that there was a fair level of prejudice at that place, so he in no way volunteered that he was gay.
The two of us worked back one night, and were having a chat. Somehow girlfriends came up in the convo, and he said he didn't have one. I then asked him if he had a boyfriend, which I wouldn't normally. Anyways, was all good, but later he wanted to know how I 'spotted' him, since he was trying not to be spotted.
I still don't know, but the closest I could guess is that there were some things he
didn't say, rather than did. Maybe most guys would say they were single, or...I dunno. Probably a bunch of small things adding up to a lucky guess on my part. I'm kinda thinking 'gaydar' is really just taking all the various cues available, and determining based on them. Some will be better at it than others, and some things will be on a pretty much sub-conscious level.