I'm sure someone far more erudite has already said this, but in case not - what it seems like to me is that there are two issues here and men with an agenda are trying to conflate them so as to yet again suppress women (told you I'm not erudite).
Transexualism in the old-fashioned use of the word, ie people who for whatever reason want to live as the opposite sex (you can't change your sex, but you know what I mean). Many undergo medical treatment, surgery, get a GRC etc. I, and I suspect most of the people being accused on here of "transphobia", would have no issue sharing a bathroom with these people, calling them by their preferred name, using feminine pronouns for them. These people have a shite time of it from society and deserve support, compassion and protection.
Self-ID that allows any bloke that wants to, to call himself a woman and access women's spaces, while at the same time saying a bloke for all intents and purposes. I'll be damned if I'll class these people as the same as the above, but these blokes keep framing the debate in such a way that you are made out to be a transphobe if you oppose self-ID.
What I see in the trans threads on Mumsnet is people opposing the latter group and the negative effect self-ID will have on women's rights, and getting told that means they hate the former group. Which is not true because the two groups are not the same, no matter what the blokes in the latter group would have you believe.
In short, support your trans friends and family and don't fall for the paternalistic bullshit of the self-ID group.