Decline for the same reason you’d decline a Mazola party or a satanist wedding.
Besides, you’d be celebrating something that doesn’t exist. There is no such thing as gay “pride.” Arrogance maybe. Justification yeah. Indulgence for sure. But who could be proud of being a bun-buggering deviant?
A Mazola party?
(Don’t tell me if it needs to be described with vulgar words.)