And yet, gay men can be sympathetic to the needs and experiences of trans women.

The LGBT community is, in part, built on bonds of similar experiences of ostracism by the larger society.

Why not add other marginalized group to LGBT if being marginalized is the only criteria?

This is what you often see LGBTQ+ (the plus means anyone marginalized). Yes, it typically has to do with gender or sexual differences but that is pretty much marketing. It is easier to organize if you have a relatively clear community.

Groups have always had ways to define who was part of the group whether that group was political, religious, or a group of gamers. Calling it "marketing" sounds a little dismissive.

I said part of the reason for association was a shared experience of ostracism. Another is being outside the mainstream in a sexual and gender sense.

In recent years, there have been some efforts to tie racial justice into the LGBT identity, but that has been controversial as there is less alignment around issues. Black and brown bands were added to the rainbow flag as part of this, and that was also controversial. The feeling was that that flag was not originally racially focused.