This is a very problematic choice and as much as I want to think it wasn't malicious, at every turn it sure looks like it's meant to be inflammatory. I can think of exactly one good reason to mark religious content as NSFW (under F-Droid's bizarre and very not normal definition of that word): To protect persons living in areas of the world where association with that religion is ruinous or outright dangerous due to p…
Abrahamic religious texts, and a lot of others as well, are offensive. They clearly and directly glorify oppressive and/or genocidal violence in the past. There's a very strong argument that they demand similar violence in the present and future. They definitely demand a whole bunch of evil and oppressive social institutions. They're more offensive than hardcore porn. Any "believers" who claim they don't really mean what they say should get exactly as much consideration as people who claim hardcore porn doesn't really mean the sex.
It's just that F-Droid shouldn't be in the business of caring what's "NSFW".