why would anyone think rainbowwashing is sincere and not just a form of opportunism attempting to exploit queer movements

In some cases it's also an effectively free concession to pressure from workers internally. Like I know of one tech company in particular - through knowing some people who worked there at the time - where the little pro-LGBT stuff they'd occasionally do was the result of queer employee groups pushing for them internally. It's still an empty and cynical statement, but it's avoiding internal tensions instead of courting public approval.

That is a common phenomenon, but there are definitely individual members of the bourgeoisie that are committed allies, in a liberal sense of course