Aaaaargh I do not know how many times I can say it is not about his thoughts! Please stop explaining to me that men have sexual urges, for Christ's sake. I am not an idiot.
Ok. I'll try again.
Yes, men (and women) have sexual fantasies of all kinds of natures. There will be men working in schools who fantasises about teenage girls, men fantasising about their colleagues, there are men who fantasise about incest, underage girls, there are men fantasising about dogs, there is no end to what is in people's heads - some of it odd, some of it unsettling, some of it enormously depraved. I get it. Whatever.
Sometimes those fantasies become actions. When someone having those fantasies translates it into action, that can be a problem. When they cross a boundary, it's an alarm signal.
Sometimes these boundaries cross a legal barrier - downloading images of child abuse or bestiality for example. Sometimes they cross a workplace barrier- sexually harassing the colleague they've been fantasising about maybe.
The boundary here is subscribing to a recently ex-pupil's sex site, making himself identifiable to her, using the school address - all exacerbated by the fact she's actually producing schoolgirl content. This is the problem. Not his thoughts - his actions.
Once a boundary is crossed, it becomes easier to cross the next one.
When people are in positions of power and trust, the bar is higher. Think about the Oxfam staff who hired sex workers in disaster zones for example. No doubt people here would argue 'they were buying a service that was being sold'. They were still sacked for gross misconduct.
A teacher is buying schoolgirl themed sexual content from a pupil he taught four years ago. His behaviour is a problem. He is potentially a safeguarding risk. Not because of his urges or his private thoughts but because his behaviour has crossed a boundary, the moment he put his name and payment details in.