"You're taking this too seriously, dear."
It's a common criticism of feminists—that we have no sense of humour, that we take things way too seriously. In this case, some months ago my mother was telling me about a distant cousin of ours who had male-to-female sex-change surgery. She still refers to that distant cousin as "he," and even laughed at me when I corrected the pronoun to "she," telling me I was taking it too seriously.
Um, no. Because this is someone's life, and it is something that should be respected and taken seriously.
It's not like trans people suddenly wake up and decide that they don't want to be the gender their bodies are shaped like/were shaped like at birth. It's not like it's a silly little game of dress-up, or like they'll eventually "come to their senses" and decide that their bodies are the right shape after all! Whatever the reason for it is, these people feel that the usual means of determining gender have failed them, and they're doing what they can to be the people they feel they really are inside. Given the importance that gender has for most people's identities, this is absolutely important. And it shouldn't depend on whether we cis people think they have (or have always had) the right genitalia for their gender presentation.
Unfortunately, to avoid ruining the rest of the evening I had to just shut up. But inside, I was furious. One does not refer to a trans woman as "him", nor does one refer to a trans man as "her". It's as incredibly hurtful as mocking fat people for our size. Maybe even more so. And those of us who are not trans do not have the right to define trans people's lives for them. Referring to people with gender pronouns that they feel do not adequately describe them is crossing a line of indecency that nobody should cross.
And saying so does not mean I'm "taking this too seriously." These are people's lives we're talking about here. This is an issue that's caused a lot of pain—an issue that has cost many trans people their lives. And if we who are cisgendered don't recognize this, or if we downplay it like it's some kind of joke, then we're actively harming a lot of people who don't need any more of this garbage thrown at them.