Puzzling isn’t it!
Well at least she’s into watching porn and using toys. There are many wives who won’t even go that far.
I don’t pretend to be able to fully understand what’s going on with my wife much less yours but it sounds like there’s a bit of guilt tied up in there.
It’s great as a fantasy or when it just happens but the moment it is more concrete it becomes something to be ashamed of, even more so if, perhaps she’s thinking of it being two other men and not you and another guy.
Communicate and talk it out if you can. That’s my best advice.