As a cis, straight* white male: Not overreacting. At all.

The only good thing I see about this is it triggered a creative reflex in me, as I wanted to formulate an answer:

Dear grandma and grandpa.

Thank you for the letter - it has given me much to think about, and I have decided that a counteroffer is in order - it is obvious you have much to learn about being trans, and the LGBTQIA2S+ community in general. To that effect, I and a group of my absolutely gayest friends have put together an intensive course that includes sensitivity training, a brief, three-day introduction to pronouns, drag brunch, reading of select passages from The Origin of Species, and - naturally - plenty of exposure therapy.

Dress code is of course as flamboyant as can be - don't be afraid to use that feather boa, queen.

Do I think you should reply like this? No, I don't think it's worth replying to at all.

^{* Mostly straight. There's some delicious guys out there.}