or
Just stay away from dodegy 3rd party apps and app stores.(or, if you must use them use them on a device you don't care about)
Get your porn on your computer and copy to phone if you need it mobile.
Outside of say china I've yet to read about any mass outbreaks of malware on android (nothing compared to the likes of windows or wordpress etc)
I'd love to have an up to date(security) phone but unfortunately most times that would mean a newer OS when I'd rather just have a patch to fix the issue. Last I saw on el reg google was still releasing patches for 4.4.x.
But of course ATT would rather me buy a new phone when I like my note 3 a lot (more than note 4 5 and 7, though 7 sounds like a collector's item now)