hmm...
This is a tough one because honesty is so important in a relationship and when you feel that your partner is hiding or lying about something, it makes the rest of the relationship suffer.
So here's my question for you.
If he did sleep with your mother once, would you leave him over it? If they're still having an affair would you leave him over it? Or would you want to work things out either way?
Look I believe that cheating is a symptom, not the cause. When one partner is cheating, there's always and underlying relationship or personal issue. Many times it takes couples counseling to get to the bottom of it and work through it. And yes, it is work to get through it.
Many long married couples have issues of cheating in their history and they found a way to work through it. Many are unable to...
My concern here is not the actual act of having sex with someone, but more the fact that there may still be some dishonesty around it. The story that your husband is telling seems a bit 'imaginative' to me and unlikely.
If I found naked pictures of Dan's mom, the first thing I would do is tell him - OMG, You're not going to believe what I just found!
Your husband hid them... In the bathroom... where many men go to masturbate... This story just doesn't add up to me. It could be that he just has a fantasy about your mother (or mother figures) and uses those pictures to explore his fantasy. What guy doesn't have a MILF fantasy? Maybe he is embarrassed - especially if you found the pictures.
If you really want the truth here and you have to ask yourself if you REALLY want the truth, you have to make him feel comfortable telling you anything. No judgment. No anger. You have to really accept whatever he tells you and not react. Otherwise, he's going to close up and you may never find out the truth.
Arguing and fighting with him about this will not help your relationship and neither will holding onto all of this mistrust toward him.