The telecoms already have that type of data that could be leaked to an extent. In 2013 British Telecom enabled porn filtering [1] that was on by default, so when setting up your account you'd have to opt out.
This was present on mobile networks for a long while. The honest and real reason I had it disabled was when it listed the ThinkPad knowledge collector http://thinkwiki.org/ as adult site... ( I'm not joking. ) - this is probably also a prime example lists like this are complete and utter jokes.
But do they have a record of the reason you disabled it or did they just change a flag and lump you in with everyone else?
A bit more paranoid. If they do this tracking for porn, maybe in the future they add other types of content they want to track. I know it sounds silly but it's not like the data is going to go away and likely will only ever increase the amount of tracking. Sadly I know many people who see no issue with this.
At that time, I was probably listed as requested access to mature content; reasons were not required but a few years ago they were already asking for identification, eg. passports.
What's frightening is how silent everyone is about this.
Opting out was such a humiliating experience. I had to spend 30 minutes on phone with a woman to explain why I needed it disabled and why I don't have a british driving license.
Luckily the filters always included way too much stuff, so calling to have the filters off wasn't ever a conversation about "I want to watch porn", but "I want access to this website about testicular cancer".