It’s a common anti-trans argument online.
Man and woman are genders. Gender is a social construct. Social constructs can change.
The binary norm is recent and only has such prevalence due to colonialist empires imposing it everywhere.
There are both extant and extinct cultures that had this all worked out already. It’s annoying that Western culture refuses to draw from those cultures (or even acknowledge them) as it struggles to catch up with the cultures it previously destroyed while praising itself for being progressive.
That is not an argument. It is a statement. It requires supporting evidence. If they are referring to the essentially notion of gender roles maybe they are saying that people that are assigned as one gender at birth will never question or desire to live as another gender, but clearly many people do. In fact many people change their appearance at the cost of their financial and social well being, so I find it difficult to believe that this is anything other than something internal to the trans individual. Also many other societies in the past had different of gender roles that were more accepting of trans people so this is not a novel invention.
I don’t understand why anyone would say this kind of “men can not be women” and citing a dictionary (PURE IDEOLOGY) as a justification of what clearly is happening can’t be happening. If you want to engage with them, just ask them questions in a Socratic method. If they don’t want to learn, you are going on an uphill battle that might not be worth winning.
Arguing with people online is generally not a dialectial process. At best, just spread propaganda to treat minorities with the same respect that you would a non minority doing the same thing. If they can’t grasp that concept, perhaps you are arguing with a fascist that finds the basic human decency taught on sesame street and other kids shows antithetical to their values. There is no shared ground between us and them except the material world. Thus, you should build something good if you can and not concern yourself with these ghouls.