What motivated the Indian supreme court to legalize homosexuality?

Question

According to the Pew Research Center article Key findings about religion in India, 97% of the Indian population is religious. Now, in all of the popular religions like Hinduism, Christianity and Islam, homosexuality is seen as a sin/unnatural, yet in 2018 homosexuality was legalized in the whole of the country.

I simply can not understand how this happened, because in my experience, people who are religious hold very tight to their beliefs, so how was such a liberal ruling instilled unilaterally on the whole country?

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Some commenters found it controversial that I said Islam, Christianity and Hinduism prohibit homosexuality. Here are the relevant Stack Exchange posts:

• Christianity on homosexuality

• Hinduism on homosexuality

• Islam on homosexuality

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Some people try to write this matter away by arguing that India is secular according to the constitution, for them I bring up the following facts:

• India has laws restricting funding from abroad for certain religions, including Christianity. See here for instance.

• India has laws restricting dietary traditions on religious lines. Eg: Beef ban

• India has blasphemy laws, 'hurt sentiments' laws. See section in Wikipedia specifically under laws restricting freedom of expression

• India has laws making it harder for inter-faith couples to marry/cohabit. See love Jihad laws

• Indian governments sometimes build temples, fund temples, and manage temples [places of religious worship, not institutions like schools/hospitals]. See funding of Ram Mandir temple, many other examples exist.

On paper it is said to be secular, where the law is independent of religion, but the facts suggest to me otherwise.

Answer 1

Because the 1860 law that criminalised “unnatural” sex is unconstitutional

In essence, Navtej Singh Johar v. Union of India decided that criminalising sex between consenting adults violated the Constitutional right to equality. You have a Law in the Constitution that says people have to be treated equally. You have another law that says people in same-sex relationships are to be treated differently. The law in the Constitution wins.

According to the BBC:

Thursday's decision was delivered by a five-judge bench headed by India's outgoing chief justice Dipak Misra and was unanimous.

Reading out the judgement, he said: "Criminalising carnal intercourse is irrational, arbitrary and manifestly unconstitutional."

Another judge, Indu Malhotra, said she believed "history owes an apology" to LGBT people for ostracising them.

Justice DY Chandrachud said the state had no right to control the private lives of LGBT community members and that the denial of the right to sexual orientation was the same as denying the right to privacy.

The courts in India, like they are in most common law/civil law jurisdictions are interested it questions of law. Questions of sin and morality they are happy to leave to religion.