So there's a new season of She-Ra...

[Reddish Mage]

I think there is a distinction here. Bow liking girly things at some point in life is not specific evidence he is trans. Bow taking action amid trans-pride colors, putting on clothing that looks like something a FtM would wear, or being the central character in an episode that contains specific elements of a trans-coming out story (I thought it was a gay-coming out or a gender-queer coming out story) are all very specific evidence that Bow is FtM.

Bow is not pre-transition MtF. There’s no indication that Bow is uncomfortable with his gender-identity, or he wishes to change another aspect of his identity now. He’s definitely a he, even if he likes or liked some girly looking things.

It's more than just that of course, but I can't find the masterpost of it. I know the fact that "Reunion" is really heavily coded as a gay/trans coming out story really helps it.

Also also, just as a sort of Thing; the main show-runner has made it explicit over twitter that cishet is not the default in the show. Never assume someone is that unless explicitly shown.

Reunion could mean Bow is trans, or gay, or it could just be used as a way of drawing a parallel of his situation to that of his parents, who are gay. Finally, it could just be a message to the audience, who knows its a coming out story, even if there is no in-universe equivalent to “coming out LGBTQ.”

The fact Bow wears feminine colors and apparel carries meaning in our culture. This could be a statement about Bow being some form of gender-queer (or it could be a culturally subversive act by the authors or simply an attempt to honor the original). I’m not sure what the reason was for the design either.

Bow is Trans is a theory, whether it is true or not doesn't matter...

Getting frustrated by this fan theory specifically is a silly thing considering how many shipping and other fan speculations there are.

Bow being trans is not merely a fan theory if they are using codes. Codes have a meaning, its just only known by a subset of the audience. When fans say Bow is being coded-trans or gay they are saying the authors are very purposely using imagery to let us know a certain message.

Some codes are more obvious than others. It’s quite clear what it means when Captain Marvel changes from Kree colors to red blue and gold. It’s not subtle why they introduced Thanos by having him be told “to fight the Avengers is to court death.” However, in the latter case we didn’t know whether it was meant to be a symbolic reference to the comics or to foreshadow that the literal personification of death was going to show up.

This is not a case when fans are simply speculating on something that the narrative might tangentially support, nor is it the product of some deep literary analysis. We are dealing with themes that the authors planted and also are telling us they are planting in the show. These are substantial elements of the show we are talking about.

As for "why can't he just be a boy who likes girly things?", I can say that... I mean yeah, that's true. We need more of that in media as well, you're not wrong. But ultimately "why are you the way you are?" is a weird question to have. There's no real structural reason why he's trans because there are some things that people just are and that's okay.

I’m understanding you to say that Bow could just be a cishet that likes some girly things...but we are being given many signals he is something else, and that’s the point.