Peter Spooner said the remarks at PMQs - as Brianna’s mother, Esther, watched from the public gallery of the House of Commons - were “absolutely dehumanising”.
Tory MPs have also broken ranks to condemn the prime minister, who has so far refused to express any remorse.
Peter Spooner told Sky News: “For the prime minister of our country to come out with degrading comments like he did, regardless of them being in relation to discussions in parliament, they are absolutely dehumanising.
“Identities of people should not be used in that manner, and I personally feel shocked by his comments and feel he should apologise for his remarks.”
Sunak made the remarks as he launched an attack on Keir Starmer for changing his policy positions.
The PM said this included “defining a woman”, before adding with a laugh: “Although in fairness that was only 99% of a U-turn.”
That was a reference to Starmer’s previous claim that 99.9% of women “haven’t got a penis”.
In response to Sunak’s comment, Starmer hit back: “Of all the weeks to say that. When Brianna’s mother is in this chamber. Shame.
“Parading as a man of integrity when he has got absolutely no responsibility.
“I think the role of the prime minister is to ensure that every single citizen of this country feels safe and respected and it’s a shame the prime minister doesn’t share that.”
Esther Ghey later met with Starmer, who said he was “utterly in awe of her strength and bravery”.
At the end of PMQs, Sunak said what had happened to Brianna Ghey was “an unspeakable and shocking tragedy”.
He added that Esther Ghey “deserves all our admiration and praise”.
But following PMQs, Sunak’s spokeswoman doubled down on the prime minister’s comment, insisting it was “legitimate to point out the number of U-turns the leader of the opposition has made”.
Former minister Dehenna Davison, the Tory MP for Bishop Aukland, joined those who have condemned the PM.
She said: “The debate around trans issues often gets inflamed at the fringes. As politicians, it’s our job to take the heat out of such debates and focus on finding sensible ways forward, whilst ensuring those involved are treated with respect.
“Given some of the terrible incidences of transphobia we have seen lately, this need for respect feels more crucial than ever.
|That’s why it was disappointing to hear jokes being made at the trans community’s expense. Our words in the House resonate right across our society, and we all need to remember that.”