Actually, allowing gay sex between women was a 2007. revision of a British law.

https://en.wikipedia.org/wiki/LGBT_rights_in_Singapore

>Singapore law inherited from the British Empire prohibited sodomy regardless of sex. As such, heterosexual and homosexual anal or oral sex was illegal. In 2007, such sexual activity was legalised for heterosexuals and lesbians, but not for gay men.

Indeed the law was reviewed in 2019 and the government didn't see anything wrong with gay sex between men being illegal.