Great question! My husband has ended his affair (as far as I know & can tell), and the few times we've been intimate since, I do feel lonesome afterwards. I know he's still "grieving" that God awful "relationship," but my desires for him are still very real and have only increased over time (before I knew of the affair, or any dissatisfaction with the marriage). What does "safe" look like when you're in that limbo of the healing process?

Putting aside debates about the morality of masturbation in any context, I will add that it actually is possible to masturbate without fantasizing. It is more difficult, but it can be done while just focusing on the physical sensations, pleasure and release.

Also, every sexual experience you have shapes you, creates pathways and trains your sexuality. The more you masturbate to fantasies, the more your body/brain needs fantasies to orgasm, eventually even when having sex with a person you will rely more and more on mentally checking out and fantasizing instead of focusing on the actual, physical sexual activity. I think this is unhealthy, even if the fantasies are only about your spouse, even if the fantasies are only reliving past encounters with your spouse or of things your spouse would consent to in real life- you are still orgasming to images in your mind instead of to sex with your spouse. And at that point, you may as well be masturbating, because you aren't connecting to your spouse through sex. They are basically just your sex toy while you fantasize.

Also, to me, fantasizing about someone who has not consented is akin to raping them in your mind. You are taking sexual liberties without their consent. An extreme opinion, but my opinion nonetheless.