Here you go, then. That's the first hit when Googling "john byrne transphobia", but there are plenty of others to choose from.
If Byrne's work means a lot to you, obviously you're not obliged to do anything in response to that information. My disappointment in Byrne as a person hasn't stopped Northstar from being my favorite character. (Funnily enough, I wouldn't know enough about Byrne's views to be disappointed in him if I hadn't liked the character enough to do in-depth research on his origins.)
But you'd be wrong about Byrne being a long-time ally. His views on gay men have certainly progressed since the days when he disparaged another artist's work by describing the men he drew as "queer and slimy"; he ripped John McCain a new one over his stance on DADT almost a decade ago, and that was heartening to see. He's also said he doesn't feel his treatment of Aurora was respectful or accurate given what he knows now about DID. But those were a long time coming, and progress on those fronts doesn't excuse willful ignorance and bigotry on others.