Transphobia is enforced by patriarchy. Patriarchy is an inherently extractive system that exploits the unpaid or systematically undervalued labor of women. Thereby all communism is necessarily anti-patriarchal. And because feminism can only be fully realized without capitalism, all effective feminism is also communist in nature. Trans rights are the cause of the vanguard of antipatriarchal activism. Trans rights can never be fully achieved in a system that exploits women, as the most vulnerable parts of the community are trans women, and it can never be fully achieved under a system that makes healthcare a commodity as a sizeable number of trans people have medical transition as a basic humanitarian need. It follows from this that trans people, particularly transfeminized people, are a revolutionary class, as our vital interests can only be fully secured under a non-capitalist, non-patriarchal system, which is obviously communism.

Modern communism struggles against the capitalist patriarchy and misogyny that drives transphobia, fascism as a tool of the capitalists uses scapegoats like trans people to divide the proletariat and instigate violence from reactionaries.

There isn't necessarily a connection between gender and communism, but in the current political landscape the dominant leftist views in the west are pro-trans rights, as they are pro human rights in general. I wouldn't trust a communist org that wasn't explicitly supportive of trans people.

Capital maintains every marginalization under its purview. It serves a purpose: a scapegoat, a focus, by which to frame and maintain a conflict other than the one staring virtually everyone in the face: class conflict.

The marginalizations are real and so is the hate, and they often have legacies extending back through history before capitalism, but if it were so against capital's interest it would be torn down rapidly. And if it weren't in capital's interest then those at the top, disconnected from the common person, would not be trying so hard to stoke it, and throw money at it, and it would not have its modern character.

For transphobia in particular, there are many intersecting facets of capital that collaborate, systemically, to define and reinforce it. The most direct is, of course, the attempt to invalidate the very fact that being trans exists, that it is something that people are. Being trans challenges gender roles that came into full modern fruition under capitalism and are presented to you as having always existed, whereas it is often primarily an expression of Eurocentric domination, particularly Anglo domination, following the path of colonialism to imperialism, the development of capitalism itself. To challenge the gender roles is to challenge the various interlocking forces of capitalist conservatism. The capitalist-friendly churches. The vast marketing schemes around gender. And, of course, patriarchy itself, which requires a gender dichotomy in its current form. The oppression of trans people is inseparable from the patriarchy and therefore the oppression of women and it is why trans women receive the greatest focus in reactionary media and discourse. And the patriarchy is itself reinforced by capital, and the capitalists themselves are patriarchical.

But the transphobic oppression by capital doesn't end with the direct and obvious. The marginalized don't just face the direct oppression, the marginalization and oppression compounds against the primary (i.e. greatest forces of) oppressions of the capitalist system. Marginalization means less ability to rely on community - and capital disrupts community, forcing workers to regularly move and to rely on nuclear family structures, even abusive or neglectful ones. When the marginalized face oppression, it is harder for them to get assistance than someone who isn't facing oppression! And marginalization coincides with class descent. Poverty. Lack of healthcare. Lack of housing. Every advocate against guaranteed housing is fighting disproportionately against the well-being of all marginalized people, including trans people. Trans people are targeted directly with restricted healthcare but also systemically due to that class descent: being unable to afford gender affirming care, or any healthcare, has the exact same medical impact as being banned from receiving it.

Another hint is to just pay attention to your enemies as someone who supports trans rights. Not just the bigots who shout slurs at your rally or canvassers for a policy change or something. But who actually makes those social changes directly difficult. Who funds your direct opposition? Are your supposed political allies (liberals) actually fighting or are they making excuses? Are they fractured, and if so, by whom? Why aren't they being thrown out of the so-called "progressive" party? Who says there aren't enough funds for building community centers, housing, etc to help those marginalized by their families, targeted with violence? Who funds the more obvious opposition, the reactionaries ("conservatives")? Why do the cops protect the transphobic harassers and attack the pro-trans organizers? Why do judges recognize fascist harassment and violence as free speech and self-defence but advocacy in the streets for trans people as "vulgar" and illegal? Who gets chosen for prosecution and who goes home?

The greatest and most visceral lesson is taught by fighting your hardest against oppression and learning where your opposition originates. Why your job is targeted when you stand up against oppression. Who is a comprador, paying lip service to trans rights but reinforcing the structural basis of their oppression and making sure you get arrested when the next Stonewall happens.

Communism is based on solidarity and equality; it's pretty much in the name

are you asking why trans people should be communists xor why communists should support some of the most marginalized people?

Nah you're right, we should spend more time defending the right of every marginalized person to be an unwilling cog of the Marginalized Person Devourer 9000.