An email arrives from a member of the Scottish Greens. I say “member of the Scottish Greens”, I should really say “former member of the Scottish Greens” because, along with several others, they’ve just been expelled from the party with immediate effect. When I tell you what they’re alleged to have done, you’ll see, if you haven’t seen it already, the troubling state the Scottish Greens are now in.

The email lays out the events very clearly. Last year, a group of members and supporters signed a declaration calling for discussion in the Greens about its stance on women’s rights. Among other things, it said sex should not be confused with gender and that women and girls should have the right to maintain female-only spaces such as changing rooms. But signing the declaration has now been judged as a breach of the party’s code of conduct and any member that did so has been expelled forthwith.

The newly expelled Green member who emailed me also sent me a copy of the original complaint and I’ll take you through it because it’s a classic example of the sort of behaviour and dialogue that now surrounds this issue. According to the complaint, the declaration on women’s rights was “abhorrent” and an “attack on transgender people”. It also says the signatories should be expelled to protect “members’ safety” and alleges the rebels broke the rules against prejudice and bullying. Finally, horror of horrors, it says they were given the chance to retract their wrong opinions and refused to do so.

You’ll probably be familiar with some of these accusations already because they’re often made against people with opinions similar to the ones outlined in the declaration. For the avoidance of doubt, what the document says is that women and girls are subject to discrimination on the basis of their sex; it also says women have the right to maintain sex-based protections, the right to freedom of belief and expression and the right to discuss issues which affect them without being abused or intimidated. It is this that’s being called abhorrent and an attack on transgender people.

Exactly why the leadership takes this view isn’t clear from the letter the members were sent; it merely states that the party has concluded they acted in violation of the code of conduct and are therefore being kicked out; they are now non-members because of their non-acceptable views. I’ve tried to speak to the Greens about this sort of thing in the past but I always get the same sort of response: no details, no discussion, only repetition. One spokeswoman gave me one sentence: “we support the rights of our trans siblings and stand against all forms of prejudice”.

The consequences of this – “we’re right and anyone who disagrees is prejudiced” – are going to be obvious to anyone who’s concerned about the issues raised in the declaration, particularly freedom of expression and discussion. One of those who signed it, and therefore one of those who’s now been expelled, is Helga Rhein. I spoke to her at the time of the declaration and spoke to her again over the weekend and I think it’s worth quoting what she says at some length because not only are her opinions reasonable, the way they’re expressed makes it clear how wrong the critics are to use words like abhorrent and prejudiced.

Ms Rhein spells out the situation in plain terms. The people who’ve been expelled from the party, she says, believe that human sex is not changeable but that discussion of the issue – whether males can change into females or females into males – has not been permitted in the party for several years. As its former leader Robin Harper said over the weekend, this is a problem: long gone are the days when policy was discussed with openness and everybody’s opinion counted, he said.

Ms Rhein makes another point. She does not deny the right of trans people to live their lives as they wish, but does object if men want to enter spaces for women, by which she particularly means spaces which have been designed for vulnerable women. She also objects to men taking part in women’s sport, or men being counted in statistics wrongly as women and says she doesn’t understand why the Green party can’t see the conflict between the rights of women and the rights of trans women. Trans women have a right to live like women, says Ms Rhein, but they should not be defined as being women.

Read more: Mark Smith: Housing emergency? Here’s something we could do right now

Read more:  Mark Smith: A message to the Greens: you do not speak for gay people. Stop it

It’s clear lots of people, probably a majority of the population, agree with Ms Rhein and the other Greens who’ve been summarily removed from the party. But what’s also disappointing, shocking in fact, is that the party does not seem to respect or recognise basic principles of freedom of expression and discussion. Their code of conduct talks about “political discipline” and that no member should misrepresent or undermine the policies of the Scottish Greens. But that cannot and should not preclude healthy, open discussion within the membership about controversial issues.

Speaking to The Times at the weekend, Mr Harper put it this way: “it is not old-fashioned to say that listening to other people matters, that freedom of speech matters, and that opinions can and often should be changed through rigorous discussion and debate.” He also pointed out that an established political party now appears to take the position that freedom of speech is denied to some of its members, mostly women, and that they will be thrown out the moment they challenge party policy.

Ms Rhein has obviously seen the consequences of this position for herself, up close. She is dedicated to the ambitions of the Green party on the environment, but she believes that in order to fight for a better and greener world, we urgently need respect and tolerance for different viewpoints. She also said she feels disappointed that the Green Party does not allow discussion about the implications of changing gender but simply expels the members who speak up for women’s rights.

So who wouldn’t agree with Ms Rhein and Mr Harper on this? People who believe there’s absolute truth to be found in politics, perhaps? And that anyone who rejects that truth must be removed, expelled, rejected? People who think they’re right and do not recognise that robust, respectful discussion can often get better results than shutting debate down? Or people, perhaps, who are so used to talking within a closed circle of mutual agreement that they’ve started to see any disagreement as a sign of prejudice or bigotry?

As another of the expelled members, David Jardine, puts it, whatever your position on such issues, expelling people who do not agree with you is a shabby way to treat long-standing activists and a gross over-reaction to a straightforward request for dialogue. Sadly, though, it does look like the leadership will not be budged and that, for the moment, the dialogue’s been shut down. But I'd say remember this: claims of bullying or attacking transgender people will only shut down debate for so long. Because that’s how free speech works. In the end, it wins.