AwkwardlyCoherent

Sorry about formatting, I'm new here, if somebody can direct me to a tutorial on how to post so that the comment appears normally, I'd be very grateful...

1) It's not "wrong", it's an opinion, a way of looking at the world, and you're entitled to it. What you're not entitled to  is rewriting reality and erasing women on the basis of it, which is what's happening right now. You personally may not be responsible for it, of course, but gender identity ideology is what drives it. And what IS gender identity? "a psychological  thing" is not a definition! What does it mean to "feel" like a woman (or a man)? What does it mean to "identify as" a a woman (or a man)? So somebody is desperate to get the body of the opposite sex, you say… Sorry, sounds like a mental illness to me and a psychologist is probably the best person to help with that. I can accept that it sounds painful, and I can feel compassion towards somebody who feels that way, but just because somebody feels that way doesn't mean they get to enforce dehumanising language to talk about women, reducing women to a set of body parts and body functions: people with uteruses, people who menstruate, birthing parent, etc, etc. That is nauseating. There is already a perfectly good word for those people: women. Why is gender ideology so keen on redefining that word? 
2) Sex is NOT assigned at birth, sex is *recorded*. In 99.99% of cases, humans are born with one of 2 sets of sexual characteristics, one set we call "male", the other we call "female". They are unmistakeably different visually, and the difference is even there at the cell level, although of course at that level it's not visible. So that's what get put into an official record. 
Then because of sex, human beings more often than not are put onto one of two "boxes" on a societal level.
A human born with male external sex characteristics (i.e a boy) gets put in the box that says (non exhaustive list) "you must be though, not express your feelings; you are the superior being and the boss in charge of everything; your opinions are always considered valid; you dress a certain way; etc etc etc etc etc… Important to note that the content of these boxes varies through history and between societies to at least some extent - for example in the 18thC, men wore shoes  with high heels, for example). 
A human born with female external sex characteristics (i.e a girl) gets put in the box that says (also a non exhaustive list) "you are sweet and compliant; you must always put others above yourself; you are emotional and unreliable, and your life has less value; don't worry your pretty little head about things, you just be pretty; you have no value besides your ability to bear children; you dress a certain way that is very different from males; etc etc etc etc etc…) Some of us call these boxes "gender roles" or "gender stereotypes", and do not believe that they are inevitable. Still, "gender" comes closer to being assigned, in that "society" places different expectations and values on humans depending on what sex they are. …or, in other words, gender is a social construct. Which as an idea is pretty much feminism 101, as far as I'm concerned.
3) I believe that was a refutal of the argument put forward by companies like Mermaids, who see a boy who wants to wear a dress, or a girl who wants to play with trucks (for example - I want to keep things simple) as evidence that their body doesn't match the gender box they should - according to Mermaids - live in (boys shouldn't want to wear dresses, girls shouldn't want to play with trucks), and so they must be put on a path of medicalisation for the rest of their lives, because of course we simply cannot have boys wearing dresses or girl playing with trucks, I mean, the horror!!!!! Some of us think that Mermaids attitude is HUGELY regressive.
4) I'm sure it exists for people who believe in it. But not all of us do. In fact, I believe most people have no idea what a gender identity is, or care about one. I personally do not have a gender identity. I don't "identify as" the sex I am, my sex is just a reality (and it was not assigned at birth, it was recorded). It matters in some ways, for example around health issues, it doesn't matter in others (thanks in no small part to feminism, or so I believe). Also, my identity - my sense of self - is made up of many many many things, and I certainly don't define myself by my sex or my "gender". Also, yes, humans can "identify as" a gender that's different from their sex i.e. they can go and live for most (most - not all) intent and purposes in the "box" of the other sex (I'm assuming that's what it means? Correct me if I'm wrong.). They can do that *precisely* because gender is a social construct, by the way, and in several countries, we have encoded this in law, and individuals who do that are legally protected from discrimination, and that is proper and the way it should be. But no, they can't "identify as" a different sex. Sex is a material reality, whether you like it or not. And there are areas where sex matters, a lot - health and sport, for example (not to mention a feminist analysis of the power relationship between men and women). I have no idea what thought processes animals have, but wow… the projection! Anyway, I've yet to see a good definition of gender identity in humans (one that doesn't invoke stereotypes that hark back to a very 1950's vision of gender roles), so maybe start with that instead of involving animals?
5) What does it mean to invalidate someone's gender identity? Since gender identity is psychological and is about what somebody feels like, and - according to what you wrote above - can change on a daily basis?