The reason it's toxic is because the Venn diagram between disposable email addresses and email addresses used for CSAM is a circle: If you accept disposable email and allow users to upload anything, you will have CSAM.
Whereas a Fastmail masked email or iCloud relay is still in the backend tied to your real account and identity which means it provides privacy generally for you but is traceable enough that it is unappealing for predators.
Why CSAM specifically? Isn’t it mostly going to be just vanilla fraud?
Disposable mail providers generate a ton of spam too, but CSAM is illegal to host, so it's pretty risky to allow them unmitigated.