It is a genuinely reasonable question, and the logic behind the policy is more practical than moral even if the word pornography carries a certain weight.
The primary reason is security and facility management rather than any particular stance on nudity itself. Explicit photos become a form of currency inside correctional facilities. They get traded, stolen, used as leverage, bartered for commissary items, and can become a source of conflict between inmates. A facility trying to maintain order cannot afford to have a category of item circulating that reliably generates disputes, predatory behavior, and theft. Eliminating the item eliminates that entire category of problem.
There is also the mail inspection reality. Every piece of incoming mail is opened and reviewed by corrections staff. Explicit images put staff in the position of handling material that creates its own set of HR, legal, and workplace policy complications. Facilities simply do not want to deal with that.
The child protection angle adds another layer. Facilities cannot always verify that explicit images do not involve minors, and the legal exposure of allowing explicit material into a facility where that verification cannot happen creates a liability that no institution is willing to accept.
As for why your husband does not bring it up on calls, that is a combination of not wanting to acknowledge the obvious and not wanting to have a conversation about something he cannot do anything about from where he sits. Most men inside would absolutely love those photos. The policy does not change that fact. It just means finding other ways to stay connected and keep the spark alive through letters and calls that describe what is waiting when he gets home.
That is what imagination is for.