Quoted on Think Theology blog:
I noticed some students parroting the line that biological sex is “assigned” at birth by doctors and parents rather than identified or recognized. “Wait a second”, I said. “Is sexual orientation innate, something we are born with?” My students nodded readily. This is well-established dogma. “And you’re also saying that biological sex is a construct, a category arbitrarily ‘assigned’ at birth?” More vigorous nods. “How is that possible? Aren’t those claims contradictory? How is possible to have an innate attraction to something that is merely a social construct?” Aha. In that millisecond, I saw a brief glimmer of light cut through the postmodern haze. Even if they quickly turned away, they had at least recognised the contradiction.
https://thinktheology.co.uk/blog/article/the_basis_is_biology
If you cannot define what a woman is, you cannot by definition define what 'transitioning' means. What are you going from and where to? What is objective and what is subjective? I know I've already brought this up before, but it bears repeating: transgenderism is a fantasy world of make-believe.
I think you are right to defend women's sport. Sex is binary, always was always will be. Even if you make some differentiation between sex and gender, a man who thinks/decides he is a woman is still a physically male despite what is going on in his head. It is still a moot point whether this changes if enough poisons have been administered to destroy his physical masculinity, but self-ID without such treatment simply means men are competing in women's sports.
For those who don't want to read the above link in case they get contaminated, another quote is relevant to this discussion, not least because accusations of 'transphobia' can be used to shut dissenters up:
“What, pray, are you?” asks the caterpillar.
“I’m a woman.”
“Oh are you?”
“Yes, at least…” I pause, suddenly unsure. “I think so?”
“Do you feel like a woman?”
“I’m not sure” I say. “What does it mean to feel like a woman?”
“To feel like a woman is to be a woman”, pronounces the caterpillar, taking a long drag from his hookah.
“But what is a woman?”
“Someone who feels like a woman.”
“But…what does it mean to feel like a woman, if being a woman is defined as feeling like a woman?”
“Transphobe”, puffs the caterpillar.