It simply means that their gender, their inner self, is male. For some that means having a penis. For others it can be as simple as that is what they are. And how being male is expressed can be different from person to person.
I am male, both in sex and gender. Personally I can't stand sports. To some that makes me less male or not male. Now I do like certain competitions, particularly competitive obstacle courses, such as Beast Master. I am rather emotive, not at all ashamed to let tears flow when particularly sad, or even happy. Again, there are those who would lessen or negate my being male for that. And quite frankly I enjoy wearing kilts. And when you boil it all down, a kilt is nothing more than what a man calls a skirt.
Despite my not meeting the standards others hold as being male, I know I am male. And quite honestly, what's between my legs is not the deciding factor. And trans people know what they are. Sometimes they don't have a word that matches common lexicon enough. For what I am, male is accurate enough. And some don't need their body to fully match their gender. Some trans can solve their GD simply by presenting as their gender. What this means is what they see as that gender. Just because you feel females should wear skirts and dresses, does not mean that a female, cis or trans, feels they need to in order to be female.
And that might be part of the problem. Gender and gender presentation are two separate things. I present male differently than some other males. For the most part, I present enough things that others associate with male, for them to see me as such, but others do not see me as such because of what I mentioned above.
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