It's also made very clear that trans women aren't welcome. Also, the overall tone just makes me think that, if I said the wrong thing, I'd be frog-marched out.
I think, Flora, that you're closer with your religios comparison. I'm sorry, but political parties do canvass opinion from a wider audience; they just do, so there's nothing for you to disagree with there. Religion, however, is based on an agreed doctrine so in that case, yes, it would be unhelpful to have non-believers attend.
Anyway, whatever, it's a free country.