The difference is that when these forms ask for religion, you get various tickbox options, which includes atheism, and a "don't wish to say" box. With sexuality you also get a "don't wish to say" box.

With gender questions they are always phrased so that obligatory belief in magic gender essence is assumed for everyone and there's no "this doesn't apply to me" option. "Do you identify with the same gender you had at birth?" as a "yes/no" question gives no brainspace to even the possibility that someone might say "actually gender is just old fashioned sexism dressed up in fancy new vocabulary, I don't have a gender and reject all sexist stereotypes thanks, I'm just a human being not a set of patriarchal ideas of identity". Asking for someone's pronouns gives no brainspace to even the possibility that someone might fundamentally object to the idea of compelled speech - it's fine to give believers the opportunity to express their beliefs if it's important to them so there's nothing wrong with questions like this existing but they need to be optional or have a n/a answer that doesn't require responders to pretend to be a believer in order to complete the form.

There are legitimate reasons where an employee will be expected to disclose their sex category.

I

That’s my preferred pronoun

EA protected category and disclosure are separate issues.

Being protected means people aren't allowed to discriminate based on it. It doesn't mean they can't ask about or can't mention it. And that is at least in part because some categories are bloody obvious. They don't need to be 'revealed' because everyone already knows. One of these is sex.

It's useful to compare with the GDPR regulations, which cover all personal data but give 'special category status to some types. From the ICO site, these are:

• personal data revealing racial or ethnic origin;
• personal data revealing political opinions;
• personal data revealing religious or philosophical beliefs;
• personal data revealing trade union membership;
• genetic data;
• biometric data (where used for identification purposes);
• data concerning health;
• data concerning a person’s sex life; and
• data concerning a person’s sexual orientation.

It is also important to be aware that some of the protected characteristics outlined in the Equality Act are classified as special category data. These include race, religion or belief, and sexual orientation. They may also include disability, pregnancy, and gender reassignment in so far as they may reveal information about a person’s health.

Note that 'sex' appears nowhere in these groups. And gender reassignment only insofar as it reveals health information.

Because we can all see (and hear, and to some extent smell) it.

Yes, this exactly. At the moment, it's set up as a variant of the 'have you stopped beating your wife yet?' conundrum.

If you humour/tolerate them by choosing to interpret their use of the word 'gender' to presumably mean sex, you have implied that you are in accord with their own beliefs - those of gender and sex being both different things AND the same thing, however they choose to frame and manipulate it.

On the other hand, if you choose 'my gender identity is not the same as my sex assigned at birth' - because you don't have or believe in a gender identity, so how could you possibly have one, and what does it have to do with sex anyway? - then you will certainly be regarded as trans/NB/gender-fluid/questioning or whatever.

As you say, without them giving an option for 'I do not have a magic gender identity - I am just my biological sex', they're deliberately rigging it so that you're forced to comply. It's no more honest or worthwhile than a company doing market research on a product and making people tick one of two boxes to indicate whether they would describe it as either 'amazing and great value' or otherwise 'wonderful and very affordable'.

At least, in the old days of pen-and-paper surveys that were counted and analysed by actual humans, you could add your own option or description of why the given options were faulty/lacking and thus your reasons for not answering - the same as with a general election ballot, where you can deliberately 'spoil' it. Nowadays, you're forced into Hobson's choice, and if you don't, the computer patronisingly tells you that YOU have made an error and will refuse to let you submit it.

You also can't be prosecuted for "lying" on these forms.

A fact that had to be dragged out of one rather irritating bright-eyed young thing years ago.