Is it normal that i prefer to masturbate?

Is it normal that I prefer to masturbate instead of fucking my sexy wife?
We have a pretty good communication and although we have been together for more than 20 years we enjoy an almost daily sex together while describing our mutual fantasies to each other. However lately I find myself more and more wishing to masturbate instead of fucking her. I feel like I'd like to do it at my own pace with some porn. Since we have sex daily I have no energy to masturbate and have a good erection for her. When I masturbate I don't have to be hard so that she will feel she turns me on. I feel that if I told her I prefer masturbation she might get insulted. So she is not aware of how happy I am when she has her period and I can masturbate freely at nights without worries.
Now that I've wrote this I understand that maybe there is some room for improvement in our communication but I'm not sure how.