I'd like to learn more about Dan Savage's alledged transphobia.?

Yeah, unfortunately Savage doesn't have a very good track record with transgender issues and knowing how not to use trans slurs or call someone trans as an insult. It's really unfortunate, given that he's one of the more visible and well-known voices in the gay community. Of course, last time I heard, he had some offensive and problematic things to say about bisexuals, too.