> Where are the upsides in CSAM - whether real or computer generated? How does it benefit society?

In a public forum like X, there probably are no upsides.

In general, though, pedophilia exists. This isn't something that is going to change. What is the harm in providing them with a alternative to real CSAM (which actually and actively hurts children)?

Giving them no legal avenue allows us to put more of them in jail. Once they are in jail the odds of them molesting children goes from "possible very low but measurably above zero" to "~0."

I think you are looking at it from the point of it being a punishment for victimizing someone -- when in fact it's used not to punish crime but to put away people who potentially might victimize someone in the future.