It could be possible that the sister's husband is starting to get dementia. My dad works in a nursing home and it's quite common for dementia patients to start expressing inappropriate behaviour, groping people etc, when they spent their whole life not doing that before.

As for your friendship - I find it hard to believe that the entire decades long friendship you had before was completely false. If he had been trying to seduce you before you would have known about it.

I think what's more likely is that your friendship WAS real during those decades, but that he misread your body language and he thought that you were sending signals that you were interested in him. Obviously you were not, and you didn't do anything wrong. It could also be a generational thing, regarding consent. In younger generations, consent is defined in much more explicit terms (enthusiastic yes, verbally saying you want something) whereas in older generations consent is often seen as more body language based. If your friend was a true friend, if he knew how much pain he'd caused you, he would be devastated and remorseful. Have you ever told him? Cause the way you describe his "we stopped cause it felt weird" statement, it seems like he did think what he did was consensual as opposed to assault. This does NOT CHANGE how you felt it and it was not OK. Your feelings of violation and betrayal are not wrong. But I do wonder if maybe you could talk to him about it and get some closure, if not your friendship back? If you knew that he had misunderstood you, would that help you feel more confident in your ability to choose friends, help you remember your history with that man in a way that's not tainted with "was that all fake?"

Not that you need to do anything of the kind, unless you think that it would be in your best interest. You don't owe him anything, not forgiveness, not friendship, not an explanation. This is only my thoughts if it was possible that he was not being malicious in his actions towards you and that it might have been a misunderstanding. (It very well might have been malicious, I don't know you or him so I am only giving ideas based on how you've written this, either way it is NOT YOUR FAULT what happened and your feelings of betrayal are valid and deserve to be expressed).

I do believe platonic friendship is possible between men and women. My best friend is a man, but fortunately for me, he's gay. We've shared a bed multiple times, and he's never remotely attempted to touch me or be inappropriate.