You can have a sincerely held belief of many things.
You can sincerely believe that it's ok to murder thousands because they are the wrong religion.
This doesn't make it a belief worthy of respect in a democratic society.
The whole point is whether that belief is a) coherent and consistent b) doesn't harm others.
Someone saying that sex isn't important when genderism itself relies on the identification of sex so you can trans, makes the sincerely held belief collapse because in law it's utterly incoherent and impossible to legislate for.
You can not legislate for fuzzy feelings that are unique to individuals. You can only legislate for ideas and concepts which have agreed, widespread consensus and can be defined clearly in a way that's reasonably understandable to all.
Legislating gender instead of sex fails in the reasonableness test for a number of reasons because of this.
It might be a sincerely held belief to some, but this doesn't make it a workable belief which recognises that society has to balance rights. The rights of transgender people to privacy, dignity and place in society have to be appropriate in a way that still observes sex - for the best interests of transpeople themselves even if they fail to recognise why it remains important.
I've used the example of doctors on more than one occasion in the last week. Doctors need to be able to talk about sex instead of gender when treating trans people. If legally you can not see sex or talk about sex only the gender of a patient, then a doctor places themselves at risk when treating a transperson if they do their job of treating the body and asking questions on the basis of sex. However if the law insists on the doctor only talking about sex and sex being replaced by gender, the patient is at considerable risk and doesn't have equal standards of health care because it's impossible to treat a patient properly without reference to sex.
Legally you can't have a half in/half out position as the Equality Act is written. You'd have to explicitly rewrite the law on equality to allow exemptions where sex was recognised in certain scenarios. That's not how the law is written. It's written with exceptions that recognise single sex needs.
The lack of ability of transpeople to understand how the law works and how human rights have to be balanced, frankly isn't my problem. It's ignorance that doesn't change no matter how sincerely their beliefs are.
Having a sincerely held belief ISN'T enough on its own. That's the problem. It also has to have the consideration of how it's workable within wider society and the impact on wider society. This isn't to say that protections are removed. They are not. It's saying that protections have to be applied in a different way - and that's where transactivists are having a meltdown because they aren't getting exactly what they want in exactly the way they want.
That's not ok. No matter how sincere they believe.
Belief doesn't change reality.