He doesn't have to say anything about trans rights at all just like he didn't have to publicly support self ID or sign any pledge.
Starmer was fighting an internal election at the time, amongst Labour members which gave a high proportion of TWAW advocates. It was an issue that was regarded important internally so he had to 'signal to the virtous' that he had a position on this.
If he hadn't would he have won the election as leader?
He may not have done.
Now the election is done, he could be a proper politician and go back on that - but he won't want to be seen to do an about face (if he indeed intends to do one which he may not) otherwise his political opponents will punish him for it. He has now started to create a situation where he could do eventually do a 'considered uturn' if he so chose. He could, equally, decide to double down. This is why keeping pressure up now is important.
Starmer has to try to balance the demands he has internally within the party with the demands of the general public which are very different to the Labour Party membership which is no longer particularly representative of wider public opinion.
I mean its really telling that the leader of a main political party cannot come out in support of the current law, which affects the rights of 50% of the population.
As it stands this is why the piggy backing of trans issues with lgb issues is hugely important.
There is widespread support for LGB rights amongst the population - both amongst Labour and Conservative voters. Views on LGB issues are aligned between the general population and party memberships.
As such until LGB issues are unpicked and separated from the T we will see this problem ongoing to a degree. The Conservatives have more leeway on this, but only to a point. (Its interesting to see trans issues trying to now piggy back BLM issues. It will be interesting to see how this partnership goes.)
The problem for women is their issues aren't critically important to either party. They are a second thought. Women's inherent problem is economic. As they are more vulnerable to economic pressures, they support more state intervention as a safety net than men do. This ultimately puts them in a position where its a choice between the least worst option when it comes to economics and the trans issue. At present the Conservatives aren't reliant on the female vote and women favour Labour because they support economic policy which harms them least. Labour need to be attracting more older males to win an election and can gamble that they can afford to take women voters for granted.
This naturally sucks, but unfortunately is a dynamic we have to recognise and act tactically upon. See my point about keeping up pressure.
I don't know how committed Starmer is to either women's rights or the trans issue. It seems its not his priority full stop. That might be depressing HOWEVER as it stands he has little actual power either anyway. He's not in government and his priority is asserting himself as labour leader to get to a position where his opinion influences the unions (he doesn't currently have that).
Johnson is preoccupied and its not a priority for him either.
So we have a few years of limbo to work in I suspect. This works to the favour of women as far as I can see, as long as we keep the pressure up. There are several issues on the verge of a legal challenge and on the verge of a potential scandal. And we have the LGB Alliance growing as an organisation.
I don't read Starmers actions as pro-trans. I don't read them as pro-woman either. But this doesn't mean it doesn't work for women either. That's up to us to work on.
I see an opportunity whether by accident or design and a window of opportunity before Starmer is ultimately forced to show his hand. And that is thanks to Starmer.