The Court of Final Appeal has reserved judgment on whether a transsexual should be allowed to wed her boyfriend.
The former man, identified as "W," had undergone sex-change surgery as well as changing her gender on both identity card and passport.
But the Registrar of Marriages says she is still biologically a male, and therefore not allowed to marry a man.
Counsel for the government Monica Carss-Frisk said the traditional concept of marriage - a union between a man and a woman- has not changed.
And the definition of a man and woman relates to their natural gender and excludes transsexuals, Carss-Frisk said.
She added the Basic Law protects the right to marry, and its scope should be compatible with social values and moral standards.
Hong Kong society has seen no change in its entrenched traditional values of marriage, so the court should not define marriage as more than "a voluntary union of a man and a woman."
Carss-Frisk said permitting a transsexual to marry her boyfriend would impact on the concept of marriage and other legal implications.
Carss-Frisk said part of the existing legislation allows transsexuals to have new life under the acquired gender.
But the nature of marriage has its special nature and should not be put on a par.
Representing "W," David Pannick argued that she has a constitutional right to wed under her new sexual identity.
Pannick said some couples cannot have children, or don't want to give birth or choose to adopt children, so it proves that fertility is not the main point of a marriage.
Pannick also said "W" is regarded a female according to her new identity card and passport, and is officially recognized as a woman in public places such as toilets.
There is no reason for her not to have the freedom to marry her boyfriend, he contended during the two- day court hearing that ended yesterday.
A pastor-friend of "W," said outside court that she just wants to justifiably marry in Hong Kong, and hopes the government will allow her to enjoy the same constitutional rights as others.