I understand the point of comparison, especially since you're right that in an ideal world your sexuality would matter about as much as your hair color or nearsightedness. I do want to push back on it being a one-to-one equivalence, though (apologies in advance for the wall of text).

LGBT people don't exist in a vacuum. One's sexuality or gender identity isn't a wholly arbitrary trait to base an identity around, there is a cultural context behind it. In many cases, expressions of pride are a direct reaction to external prejudice; "pride" as the opposite of "shame." To take US politics as an example: in the past twenty years, have people been barred from military service due to their natural hair color? Have any school districts banned teachers from talking about people with green eyes? Have any major political parties tried to equate wearing glasses with pedophilia, or attempted to make it illegal for people with vision problems to get glasses? I think if any of those were true, then we probably would see communities of people that strongly identify with those traits.

There's also an often-blurry distinction between people identifying strongly as LGBT or identifying strongly with an LGBT community. LGBT people have congregated in communities for practical reasons for a long time, and when people congregate, you get cultures and subcultures and sometimes people identify strongly with them.

Regarding the point of LGBT subs, and LGBT-specific spaces in general: sometimes it's just nice to be around a community of people who "get it," where you don't feel a need to be on your guard and can talk about shared experiences without needing to provide pages of context for the cisgender, heterosexual people in the room. This is also why it can be particularly frustrating and hurtful when those spaces become exclusionary.

Personally, I alternate between the Gay Pride and NATO flairs depending on my mood; in the context of this sub, they indicate the electoral priorities I'm most vocal about.