A man who must notify police 24 hours before he has sex.....

Well we don't know what they know he did or was involved in and often people have no convictions but are known to be dangerous. I have worked to support some people like this and without the legal restrictions that were put in place believe me they would have hurt people. Sound like they have basically banned him from accessing the internet unless he is prepared to show the police what he has been accessing, this is not that unusual when people have previously accessed illegal material etc. To be involved in sado-masochism is not a crime, to involve others who don't wish to be is, to watch others being involved in activities they did not wish to be involved in is also illegal, to coerce a person is a crime, to persuade a minor is a crime as is watching/knowing someone else coerced them. It is complicated only in the language but is generally common sense. I am presuming the 24 hr notification is so that the police can check the person he is involved with wants to take part in the activity and has the capacity to agree to it. Capacity to agree being a big definer.

He's into BDSM.

The police and courts have no concept of consent when it comes to BDSM - remember Operation Spanner.

I can only presume (we have no real facts here) that he has been involved in previous incidents where people did not give permission, were coerced or held against their will, were physically hurt and hadn't agreed to that, were minors or some other behaviour that would be deemed criminal. The article does not say he was imprisoned or found guilty of offences but he is on the Sex Offenders Register and it may be that he agreed to this to avoid a criminal sentence, the authorities may also have a view about his own capacity to consent. On that basis the judgement is not about him being stopped from continuing his sexual inclination but about protecting other people and perhaps himself who may not understand or have the capacity to agree to what he intends to do. As I said I have been involved in this kind of case involving vulnerable adults and these decisions aren't taken lightly because they do restrict the liberty of a person/s and put limitations on what is otherwise a legal activity.

I am not quoting any particular case but you do point out that some of the people in that particular case were not deemed to have the capacity to consent.  You mention consensual non-consent and I understand it in relation to BDSM, but groups cannot change or alter the law to suit themselves. In law capacity is crucial. To remove this protection would mean that vulnerable people were just fair game to any predatory person or decisions about ourselves made without or consent; we would have no redress (and many of us will be vulnerable in our lifetimes). Adults may not always have the capacity to consent; this may be temporary, fluctuating and not at all. It may be that they do not understand or that they have not been given enough information to enable them understand what is expected of them. It could also be that they haven't been given enough time to weigh up the facts and process them. The law does allow us to make unwise decisions, but this is based on the fact that we are able to weigh up the choices, predict the potential outcomes and make a capacitated decision to do something despite the risks and that we are aware of the consequences of our actions. All of these things are necessary to determine capacity and most of the time we take it for granted, but once under the scrutiny of the law capacity is a big deal. It must be and has to remain so.