My wife isn't in the mood as much as I am and she will often tell me to go "take care of business" myself. Masturbation has never been a problem with us. We both agree that when you are in the mood to have a nice orgasm, go for it. That being said, How do I keep her involved in something that I am doing myself? She chooses a toy for me to use on myself.
Each time is different and I enjoy discovering the different sensations that they offer. I have made a similar offer to her, but she has never had occasion to ask me. She takes/took care of her business with the shower massager.