I am not a dentist but generally for a medical procedure like sedation, they wouldn't want family members present. Giving any medication IV can rarely result in things like a significant medical reaction, and these things are quite stressful to manage, never mind with an audience (and in a relatively small room, which will be even more crowded as for a serious reaction, they would call for help, so other staff may be in there), plus can be unpleasant and distressing for non-trained people to witness. As a doctor, I've seen relatives grabbing the patient (so physically getting in the way of the staff trying to help), or shouting, screaming, or swearing at the staff, which adds an extra stress and cognitive load to the medical staff.
For the same reasons, when arriving at a medical emergency, we would usually ask family or friends to move out of the bay/cubicle space to give us room to work and be able to assess the patient properly when we arrive at a medical emergency call or cardiac arrest call. We can work with someone screaming at us, but we much more efficient when they aren't.
In some situations, such as a child, someone with learning difficulties or a diagnosis of significant anxiety or neurodiversity (and many others) then the patient may request support from another adult, but in your case it's just that you don't trust men.
Given that one of the well documented side effects of IV sedation can be disinhibition, and memory loss/false memory (in particular sexual fantasies), any healthcare professional giving IV sedation would expect to have a chaperone there to protect them as well as the patient - even if a female HCP and a female patient.
Essentially you've just told your dentist that you don't trust him not to fiddle with you while you're under sedation, and you are surprised that he feels a bit off with you?
My final comment would be that you might say something disinhibited while under sedation that you might be embarassed about (I've heard a woman make a comment about anal sex ), the HCP will have heard it all, but you might not wish your husband to have heard this.