Why can you discern a person's political or religious views but not their sexuality? How do you know someone is being honest about their political or religious views? You seem to be trying to make sexuality different in ways it is not.
What you are doing is conflating "self-identification" and "applying labels" which are not the same thing. Sexuality is different from religious and political views because it is intimate. I'm sorry if you don't get that. I tried to explain but I guess I didn't do a good job. The simple fact of the matter is, you don't know about a person's sexuality unless you are intimate with them or they tell you.
You have, on multiple occasions, been given examples of what purpose the labels serve. I don't know why you seem to have dismissed those. To once again give another one, if you see someone who has been labelled gay, and you are of the opposite gender, you won't try to hit on them.
Oh I know I've been given the example by ChrisL that it serves as a convenience for gossip... like, if someone says "that guy's really cute" and she says "He's gay!" And now here, where you give the example of how the label helps you determine whether to be sociable to someone. Assclap finds it beneficial to be able to apply the label to people in order to insult them or get under their skin. I'm sure there are dozens of other reasons to apply the label... like when you're renting a house or hiring an employee. So yep... lots and lots of reasons to apply labels to people. Great point!
Apparently you have no idea what the difference is between hitting on someone and being sociable to someone.
There are indications of sexuality just as there are indications of religious and political views. In every case, you can either use those indications, or a person can tell you about their sexuality/religion/politics, and also in every case, there is no real way to know if they are being honest with you. So what?
You are clearly determined to equate labels for a person's sexuality with discrimination and bigotry, yet seem to be perfectly fine with labels for other things. Again, why only with sexuality? Why must those labels be about bigotry? It's easier to say someone is gay than they only are attracted to the same gender. Saying a person is straight is not any kind of automatic insult.
People use labels and stereotypes of all kinds for various reasons. Are they sometimes used as an insult, or a reason not to interact with a person? Of course! That is true of pretty much any label, any stereotype, any descriptives used for a person. Again I wonder why you are so focused on the categories of sexual attraction and seem to consider them so very different from others? Sexuality is often an intimate thing, so what? Why does that somehow mean that words used to describe sexuality are automatically bigoted or insulting?
Yes, gay has been used as an insult a lot. So have quite a few other labels for sexual attraction. That doesn't mean those labels are inherently insulting, particularly if many of the people who fit in the labels are comfortable with the terms themselves. I said this before, but I have seen the terms liberal and conservative used as insults on this site many, many times. They seem to be considered insulting terms by quite a few posters here. Should we do away with them as well?
Oh, one last thing. What makes you describe ChrisL's example as gossip? If she knows a person is gay and tells her friend, how is that gossip? This appears to be another example of you trying to fit what people are saying into your predetermined idea of what labels about sexuality are used for.