Your parents don’t have to be very old to have grown up in a time when sex and gender were the same thing. (With the tiny exception of biologically inter-sex people). Any discussion of the increased complexity has to start with establishing that the words sex and gender are no longer synonymous or aligned with each other. Maybe you’re too young to know and no one explained it to you.
Only in 1955 did sexologist John Money first introduce the terminological distinction between biological sex and gender as a role. Before that gender was a linguistically construct and determined the article used with nouns in many languages that had gender, e.g. German and French. Previously, only things like a table or a book had gender. Words had gender. People didn’t.
And it is complicated because there is no universally accepted definition of the meaning of many terms, both old and new. The terms woman and man are now hotly debated, even if you think you can establish new definitions by posting on Medium.
From other reading on McCarthy-Calvert case, she was fired because her statement was allegedly politically motivated and intended to be exclusionary. She was using the sex-based definition of women to make a point about the definition of the word, i.e. that transmen are “women” biologically. You obviously don’t agree with her use of the term women, but it is an ancient one. You fail to recognize that not everyone agrees with your new definition, based on the narrow explanation you “taught” your parents. You explain as if it is a fait accompli and they are too old and dumb to get it, though your little brother can.
In your list of definitions, you missed agender: Denoting or relating to a person who does not identify themselves as having a particular gender. This person could be attracted to any sex, all sexes or may be asexual. Now your list is complete.
You seem not totally up on speed the new lingo yourself, e.g.: “being a cis — “a person whose gender identity corresponds with the sex the person had or was identified as having at birth” — straight man or woman”. The part in quotes is correct, but it has nothing to do with being straight. A gay person may or may not be cis. A cis person may or may not be gay. For example, a MTF transgender who is sexually attracted to women is a lesbian (though some lesbians hate that idea). You see: not cis and not straight. But if the MTF person was attracted to men, then she is not cis and straight. It’s complicated. Ask your little brother to explain it.
Even in the community promoting redefinition, people might attack you. For example, for including T in your LGBTQIA+. https://www.independent.co.uk/life-style/lgb-alliance-group-transphobic-alison-bailey-lesbian-gay-bisexual-a9169091.html
The people asking the questions are not the ones making it complicated. It just is more complicated and it is changing very rapidly. I think it is better not to patronize people, but rather accept it is more complicated. Instead of being condescending, explain how that making it more complicated is necessary to make the world a better place and explain it again and again, if necessary, without sounding exasperated. Our language is evolving to better express an ancient reality that has been suppressed. It will work itself out eventually. Sooner perhaps if we don’t alienate people.