Re: Well then...
Sex with children is, and should be, illegal. Images of sex with children are illegal.
Sex between consenting adults is legal. Images involving consenting adults should be legal.
Now sure, there is some argument that if images of non-consensual sex between adults was widespread, that should be an argument for criminalising all of them. But:
Firstly, this would be an argument for criminalising depictions of adult sex, on the grounds that they might not be consensual or staged. This law does not target hardcore porn, instead focusing unfairly on acts that might be practiced by sadomasochists. Similarly, it would be an argument for banning all violent films, in case the participants hadn't consented and people were really being harmed.
Secondly, not a single example of "extreme porn" involving non-consenting adults has ever been produced (adult sexual slavery is a serious issue, but no link has been shown between this and the "extreme porn" sites). It certainly is not widespread. We might as well criminalise any film that shows someone dying, out of fear that it's a mythical snuff film.
Thirdly, there should at least be a defence where it can be proven that the participants consented. But under the new law, sadomasochists who photograph their own acts will be criminalised (the defence for participating in the image would not apply to sadomasochists, because the law does not consider their consent valid); you might have cases where only one partner is in the image, and it would be illegal for their partner to possess - not to mention say, a threesome where two of them decide to make a kinky picture, and then give to the third. Other examples would be images from BBFC legal films, or legal porn sites that are regulated, keep records, and where the participants turn up to court to say they consented (if you are going to quibble "what if they were threatened into it" - well, again that's an argument for criminalising _all_ pornography). Also consider staged acts - it can obviously be shown that the participants were in fact unharmed.
I would be more than happy with such a defence, even if the burden of proof was on the defendant - but the Government refused to allow such a defence, because this law is not about protecting participants, but about the depictions themselves:
"So someone please explain the difference in intent between this proposed law, and the law against indecent images of kids."
The Government themselves explain this - this law is not about protecting participants, it's about the claim that looking at images turns people into violent criminals. The Government's own Rapid Evidence Assessment couldn't find any evidence of people harmed in the production of "extreme porn", and instead focused on alleged affects of viewing material. The law came about after Graham Coutts murdered Jane Longhurst, because Coutts had access certain sites - that are known to be staged with consenting adults. Please read some of the House of Lords debates:
In responding to criticisms, Lord Hunt's justification is basically that these images are "disgusting". In particular, he states:
"I do not take this very liberal approach of "If it does no harm to the people taking part, why should we worry about it?" I do worry about it, and about the access that people have to that kind of disgusting material."
When asked if a crime is committed in their production, he states: "some would be covered by offences in this country and some would not, but they were all disgusting."
Whether people are harmed or not is irrelevant, because the Government's position is to do with the depictions themselves, not how they are produced.
Also see the Government's justifications under the ECHR ( http://www.publications.parliament.uk/pa/cm200607/cmbills/130/en/07130x-n.htm#index_link_206 ) - only one of the clauses refers to protecting participants (and even there, it states staged activities - not a single clause claims the existence of non-consensual injuries); the rest are about controlling images of consenting sadomasochists; that the material "may be harmful" to viewers; that the depictions should be banned because the activities are "not considered acceptable"; and Please Won't Somebody Think Of The Children...
If you wish to believe that viewing some material turns people into violent murderers, that's up to you - but please don't think that this is about protecting participants.
Consider, can you explain why a BBFC film is exempt from the law, but a screenshot from the same film isn't exempt?