This makes for a very good sensationalist headline but I think there are circumstances where I could agree with this. Obviously, if someone has committed a very serious sex crime and there is any remote chance that they would re-offend, then they shouldn't be allowed to adopt - that's common sense. But not all people convicted for sex crimes are crazy rapists. People have been put on the register for comparatively petty things like urinating in public or trading nude images of themselves as teenagers (admittedly this was a case in America, I don't know if you could be convicted for that in the UK) - and I don't see why stupid things like that mean that a person would never be capable of being a reasonable parent.
I think that common sense and judgement is needed when deciding who can adopt rather than simply ruling someone out because of a conviction. If it's clear that somebody does not pose a risk to children, let them adopt. If there's any reasonable risk that they would, don't.