First thing, would be to ask yourself, what emotions were floating through you, that led to a moment of not trusting her.

If she is fantasizing about someone else, while masturbating, of all things, why is this a personal affront to your relationship? It's a fantasy, if that's what she's even fantasizing about, whether the person is real or not.

The point of imagining oneself with someone else, and crossing that boundary into a full blown affair..that's a far reach.

I get that you stopped flirting with other women, to be true to your relationship. I just didn't, personally, realize that it wasn't OK, for someone to fantasize about another while involved??