Why do activists always say "sex and gender are different" in biology-based discussions? (and any other questions MNers have have about pro-trans talking points)

Exactly. Gender dysphoria is under mental health issues because you can't see it. So's autism. You can't measure it empirically or truly know how it affects people, which is why people seem unable to say 'I know I'm a woman because of x, y and z, even though I am in a man's body. Come to that, how on earth do you define a woman without using biological gender. Obviously, I am a perfect example of one, but I'm willing to bet that so is everyone else here and none of us are alike. I'm not even consistent from day to day and I don't think men are either. I wonder if gender is a strong desire to be something else because you don't like what you are? Like the stage a lot of us go through when we think we have to have been swapped at birth because we have nothing in common with our parents? Then, ten years or so later, or when we have our own children, we hear ourselves and think 'help, I sound just like my mum!'

It’s a ridiculous argument and it doesn’t stand up to scrutiny. How can someone claim that a gender identity is separate from biological sex, then turn around and demand that said gender identity entitles someone to access the spaces and sports of the opposite sex? If sex and gender are not the same thing, then the classic ‘Just use the men’s room, Richard’ meme is exactly on point.

Because sex also means intercourse (sex) people often use the word gender when they mean sex. Some people feel weird about saying Sex and I’m assuming that’s why it says ‘gender’ on official documents like passports where what is meant is sex. Language issue.

I have no difficulty with the word 'sex' meaning biological sex and 'gender' meaning the socially-constructed roles etc that are imposed on the basis of sex.

The World Health Organisation's definition is
Gender refers to the characteristics of women, men, girls and boys that are socially constructed. This includes norms, behaviours and roles associated with being a woman, man, girl or boy, as well as relationships with each other. As a social construct, gender varies from society to society and can change over time.
which is OK I think.

Unlike many posters, I believe I have a gender identity, it is strongly gender non-conforming and is a very distinctive element of who I am.

The WHO definition goes on to make 'gender identity' distinct form both gender and sex:
Gender and sex are related to but different from gender identity. Gender identity refers to a person’s deeply felt, internal and individual experience of gender, which may or may not correspond to the person’s physiology or designated sex at birth.
and it starts to get a bit waffly at that point.

I don't dismiss the concept of gender, or gender identity; I think they are valid and useful concepts. But they lack the observable reality of biological sex, which has not varie[d] from society to society and can change over time as gender has.

So in my opinion, gender exists and is a valid area of study, and may be a real and important part of a person's self-image. E.g. me🙂

But that's different from sex, which is binary and immutable. People can express their gender in any way they like, conforming to gender stereotypes or rejecting them completely, or anything in between.
But their sex is either male or female, full stop.

That's what the IOC ruling has re-affirmed: in sex-segregated sports, gender/gender identity, as defined by the WHO, etc., is irrelevant, it is biological sex that matters.

Sex is biological. Gender is invented.

Sex and gender are different but it's vital that children are medicated so they can resemble the opposite sex. Well, the boys might but the girls will just be even smaller and less likely to pass as men but they are just girls so...

They will tell you that sex and gender are different and in the same breath claim that gender is a proxy for sex in law.
"Sex and gender are DIFFERENT but also my gender on my passport should reflect my special identity"

Sex and gender are different.

Sex is real, everyone has one, it cannot be changed.

Gender is an imaginary identity, some people have one; like all fantasies it is changeable.

Gender (an optional fantasy) should never trump sex (a universal reality).

It's a clear exercise at equivocation and allows them to switch between a number of definitions without clearly defining which one they are using, in order to confuse and control discussion.

It is a feature of their argument (to them), not a bug.

Personally, I think there is utility in retaining a sex/ gender distinction so that we can separate and criticise the biological from the social, but we shouldn't use woman/ man for both but perhaps masculine/ feminine for gender. But I also acknowledge that there hasn't been a distinction for long - half a century in academia, two decades in public consciousness - so it's not something I mind losing.