Same-sex relationships are a crime in more than 70 countries around the world, almost half of them in Africa.

South Africa is the only African nation to have legalised gay marriage.

"We hereby decline the relief sought and dismiss the combined petition," Justice Roselyn Aburili told a packed courtroom in Kenya's capital Nairobi, relaying the unanimous opinion of the three-justice panel.

"We find that the impugned sections are not unconstitutional, accordingly the combined petitions have no merit."

Campaigners who filed the petition to decriminalise gay sex argued that the law violates Kenya's progressive 2010 constitution, which guarantees equality, dignity and privacy for all citizens.

"We will appeal. We expect that the court of appeal will overturn this erroneous decision which in our view is very biased," said Eric Gitari, one of the petitioners, on Friday.

The justices, who began hearing the case last year, threw out the petition, saying the ban on gay sex dovetailed with broader, traditional moral values encapsulated in Kenya's constitution.

Some gay rights activists wept outside the courtroom after the verdict while supporters of the ban clapped, congratulated each other and yelled "thank you" to the judges' bench.

Aburili said the constitution still outlaws same-sex marriage but allowing gay sex would "open the door for same sex unions".

"We cannot be another Sodom and Gomorrah," Alfred Rotich, a Catholic bishop, told Reuters at the court after the verdict.

In September last year, India's top court scrapped a similar colonial-era law that punished gay sex with up to 10 years in jail, raising hopes among activists worldwide, including in Africa, for similar reforms elsewhere.

As a result of a lack of legal protection, rights campaigners in Kenya say sexual minorities are routinely abused, assaulted by mobs, raped by vigilantes or enslaved by criminals.

Australian Associated Press