Semiotics are all well and good, but let's roll up our sleeves and talk pragmatically here. There's a very practical reason why heterosexual auto-identification can be -- in certain situations -- a rather crass thing to do.
On the Internet, this sort of constant self-identification by straight folks is neither fish nor fowl nor good red herring- arguably, nothing you say here is going to have serious ramifications for your personal or business life. However, straight people also do this pretty often in "real life," where the consequences of being seen as queer are a bit more dire. What happens if you have a room full of people ALL saying, "I'm straight, but..." (gay people make great babysitters/twenty-seven of my closest friends are queer/Ellen is just the best TV show ever, etc., etc.), is that the one or two gay people in the room are automatically going to be picked out as gay just because they're not saying otherwise. So, this can actually create a situation where GAY people are saying, "I'm straight, but..." just so they aren't forced to come out.
"Lies of omission" alone won't allow one to pass for straight...in situations like the one above, one must actively tell falsehoods in order not to arouse suspicion. And although ideally, "coming out" would be as painless and offhand for queers as it is for straight folk, there are still lots of places in the world where walking around with a big pink triangle affixed to one's forehead is not such a hot idea.