Not everyone nor every guy masturbates to pictures of people they know. Not everyone fantasizes. Despite what a lot of people are saying and generalizing.
Porn is impersonal (unless it's someone known irl or caused someone to have an obsession...especially to one or multiple actors).
Masturbating to someone you know and have to be in a shared space with is different. Especially since there is pretty much daily interaction...of what sorts, you don't know.
He may be working on turning that fantasy into reality (assuming it hasn't happened).
Unless she gave him pictures, he was gone out of his way to collect pictures of her, save them, perhaps cyberstalking, and has imagined her in inappropriate ways..
Your relationship is in shambles, sex life isn't, and he's doing nothing to perhaps help repair things.
Instead (assuming that he hasn't had any dealings with other women before(doubt it)) he's zeroing in and moving onto someone else.
This relationship sounds like it has run its course.
It also sounds like having kids, quitting your job, relying on him financially has ruined things.. (if it genuinely was a healthy relationship prior to all of these things)
He is only one person. The added stress and strain of trying to keep the family afloat financially, and the children changing the dynamics of the relationship, being physically mentally emotionally exhausted , perhaps neglected by you a lot....is honestly just too much.
If you are going to stay, you need to find childcare and go back into the workforce. You have to start contributing financially and something to fall back on. ..in case divorce is in the future.
Please don't have any more children ..
especially not together. It's already a stressful, unhealthy environment in which both parents are struggling don't have a healthy relationship dynamic and overwhelmed...especially financially. The innocent people that you have brought into this, lives are impacted too.
He has checked out of the relationship. Perhaps he won't ever "check" back in..
You really don't know the level and depth of their interactions.
Are you going to wait the remainder of your life and forfeit the possibility of happiness waiting to see what happens with him?
Please if you're staying or perhaps leaving...have an exit plan, and a few backups.