Male homosexual activity is illegal even in private, and is actively prosecuted:

https://www.straitstimes.com/singapore/courts-crime/governme...

Same-sex marriages have no legal recognition and gay couples cannot adopt children. No laws exist to prevent discrimination against gay people in employment or housing.

Overheard conversations, social media posts, someone with a grudge. Figuring out whether or not someone is abstinent is not impossible.

In a police state, privacy is elusive.

It doesn't need to be a police state. Even in democracies there are nosy neighbors and plenty of horror stories due to that.

Pink Dot SG exists: https://en.wikipedia.org/wiki/Pink_Dot_SG