Would you be offended if someone shared their name without being asked? The issue here is that names and pronouns come up in ordinary conversation quite frequently.
But of-course I don't think sharing the name is rude, it's a basic personal reference. It would be rude if it's followed by "I'm straight" or "I'm a woman". As for pronouns, in most languages, you can know the [grammatically] correct gender automatically from the name. And if you don't, most grammars have middle or neutral gender, so that can be used. The issue here is that we shouldn't bother other people with our gender identity.
It’s no more rude than having baby pictures or a wedding ring or a cross around your neck. These are all examples of intimate disclosure which have little or nothing to do with some idealized cold professional relations. People wear their identity in many ways and the rest of society is perfectly okay with it.
But what if you can’t? Or there aren’t enough context clues to determine this? For example, my username contains my name in it; do you have any idea what my gender is? Or what I’d I had a “gendered” name but actually had a different identity personally?
But really the problem is that gender comes up in conversation a lot more than orientation. You can get a lot further in a conversation without the latter than the former, so some people see it fit to frontload that information.
I disagree that personal gender is often relevant in that kind of discussions (SO, or similar general topic forums), and when it is, then it's perfectly normal to share it. The same goes for dick size. But why to mention it if not necessary?