Is it normal to have violent fantasies of my husband

Let me start by saying that I have been married over 25 years to an adorable man that I love very much, we have a wonderful marriage and terrific sex life and I would never wish any harm or humiliation to him, though for the last several months I often will pleasure myself to horrible thoughts. We have several employees at our estate, a few full time and several more part time, one of them is a very handsome younger man that is a driver for us that is about the home just 1 or 2 days a week. Lately I masturbate to thoughts of him overpowering my Husband, tying him to a chair and forcing him to perform oral sex on him while our driver instructs me to slowly strip and tease my bound husband, then he forces my husband to watch us have sex and finishes by forcing his penis into my Husbands mouth again, I know I should feel guilty about this one but I do not, Is it normal to have a thought like this? And why so pleasing to fantasize about his humiliation?