Marriage laws should recognize that sexual identities can change, the Court of Final Appeal heard as a transsexual seeks to marry her boyfriend.
The former man, who is now a woman and known as "W," is seeking to overturn earlier rulings that marriage is only allowed between couples who are of opposite sex at birth.
Representing "W," David Pannick argued that as a result of medical needs, "W" underwent government-subsidized sex reassignment surgery.
She also had her gender altered on her identity card and passport.
The 37-year-old is officially recognized as a woman in public places such as swimming pools and toilets.
If convicted of a crime, she would be put into a facility for women.
She would be regarded a female victim if she was sexually harassed or raped.
Pannick said his client is now "publicly and privately" and "medically, psychologically and socially" a woman.
"W" should be recognized as a woman and be allowed to marry a man, Pannick said yesterday.
"The right to marry is fundamental ... the birth certificate is a record of historical facts."
Earlier, the Registrar of Civil Marriages ruled that W could not marry her boyfriend because her birth certificate - which cannot be changed under Hong Kong law - says that she is a man and therefore does not have a child-bearing ability.
Pannick said "W" has an artificial vagina as she underwent sex-change surgery and can have sexual intercourse.
Infertility does not invalidate a marriage, he added.
Presiding at the hearing, Chief Justice Geoffrey Ma Tao-li asked for the constitutional meaning of marriage under the Basic Law, which is the "voluntary union of a man and a woman."
Pannick said the matter in hand is whether a transgender woman should be recognized as a woman under the law of marriage.
Permanent Judge of the Court of Final Appeal Justice Roberto Alexandre Vieira Ribeiro asked Pannick if this issue should be left for the legislature to decide. Pannick replied that this was not the case.
Counsel for the government Monica Carss-Frisk said the definition of male and female was limited to the biological aspect when laws of marriage were legislated.
The natural gender should not be ignored as offspring are a core part of a marriage, Carss-Frisk said. If the nature of marriage should change, it should be changed through a legislature.
"W" launched the legal battle and sought a judicial review of a government decision to reject her marriage application in 2008 because her birth certificate lists her as a man.
She lost the initial court hearing in 2008 and a subsequent judicial review a year later.
The hearing continues today.