As a fellow betrayed; I feel for your spouse. We too have faced something similar.

I work in IT and am posting my understanding here to help others:
The reality is that the spam email, adverts etc generated by bots are based on historical behaviour on the internet. All of our clicks feed into various algorithms resulting in 'relevant'/similar content being displayed to us at a later date - days, weeks, months and years later.

This I know and it still hurts me when I see content like this sent to my spouse...it's a painful reminder and one that I need to process and chose to forgive, again.

What helped us? On his own accord, my spouse installed several content monitoring software on his devices so any new/current clicks on unsuitable content is flagged. This allows me to know that any inappropriate content that might come up is likely a result of that painful history.