This thread would be a whole lot simpler if most of the folk on it said that they were transphonbic themselves and so is JK Rowling.
Most of you are saying 'I don't like trans people and think trans people don't have the right to self determination not rights to access a bathroom that corresponds with their identified gender and lowers their risk of violent attack.'
That is transphobia.
Deliberately misgendering someone, or thinking you get to question others gender because they don't appear enough like a woman to you (as JK Rowling has done, accusing a woman boxer and various others of being trans, with no evidence) IS transphobia.
Being a trans-exclusionary feminist IS transphobia
JK Rowling using her fortune and noteriety to fund people through court who are trying to exclude trans people from spaces IS transphobia because she is specifically and repeatedly going after a vulnerable population.
Equating trans people to sexual predators or accusing them or sexual deviance IS transphobia.
The whole bathroom debate is madness because it is anti-feminist - it makes women less safe by creating a culture where we feel free to constantly question other people's gender and demand proof because they don't appear 'woman' enough to us - ie they don't conform to societal expectations of women. It creates danger for women who are deemed to have 'masculine' body types, facial hair such as through having PCOS etc.
Trans women do not make your bathrooms unsafe.
Dangerous men will do as they please regardless. They also now would be able to present themselves as being trans men using the women's bathroom, as now required by law, so you really haven't solved any problems this way!