If trans men are men, why should they be writing feminist literature?

...Because the vast majority of us lived for at least a decade as a woman, and thus have lived experience on the matter? And because men can write feminist literature in general, you don't need to be a woman yourself to understand why gender equality is a good cause, you just need to give a shit about other people. Also, many of us were identified by peers as gender nonconforming from an early age, even before consciously knowing it ourselves, and I think it lends us some valuable insight on how women police gender roles on each other.

Certainly, we often tend to have different experiences and feelings on gender than cis women do, especially if we realize we're trans early on, but you're bound to develop some relevant insight. I think cis men tend to treat us about the same as cis women, with some additional homophobia and maybe some more physical violence mixed in (because they think masculine people are OK to fight) but that cis women tend to have had very different experiences with each other than I have had with them.

But regardless, we all have certainly had to deal with misogyny at some point, if not in social situations than in the medical field, and feminism of course benefits us. I think if we're going to be affected by a movement, we deserve the ability to express an opinion on it. Whether you value that opinion or not is kind of up to you.