I think the other person is not wording their point correctly. Let me try to word it for them, at the risk of putting words in their mouth.

It is revealed to a group of 17-18 year old high school boys that their teacher does porn. These boys have access to the internet. What do you think is going to happen next? Obviously what will happen is that someone will look it up, share it, and then it causes a problem. Not because it's immoral but because you have immature horny creatures bringing something into the school that isn't appropriate. It's inappropriate because school is for learning not ridiculing or being sexist towards your teacher for doing porn.

In a purely practical sense, of teaching students with as little interruptions and interpersonal conflict as possible, it's easier to not employ the teacher doing porn. It removes a factor of friction in an already tedious and complex job.

If we lived under communism where a community of parents could take time off of their jobs and go to school with their kids, and the teacher could pause teaching, then we could ensure those kids were taught a valuable life lesson about what is and isn't appropriate, how to react to porn, and all that. But we don't live in that society, we live in the one where schools are essentially prisons that double as job training centers. Nobody has the time and we don't have the material underpinnings of an accepting culture. Thus, teachers who do porn will be fired.