Because it's not just representation. It's fetishized representation. The penis is there to inform you that the girl is trans, but also to make you feel like she's sexy because of it. Like an exciting thing to be stared at.Again keep in mind that as I said I have 0 context on CDP previous statements about the LGBT community or anything related to it, just like my friend so maybe we kinda lost because of it but still it really bugged me how can a trans person could be happy with this image and a lot of users that are not trans are so offended with it ?
A good parallel would be the fetishization of black men and their penis' sizes. Some are not offended, but even if you are a black guy with a big penis, this kind of treatment dehumanizes you and continues to objectify not only the ones who are not offended. Remember: not every person wants to be considered a sexual object, especially when this also comes with inherent dehumanization and violence. Trans people are the same: transgender pornography is not good representation. Showing trans women as sexy girls with dicks is not good representation.