My wife has always been against the idea on "her" owning a firearm but she would never tell me what to do or limit what I can and cannot buy. Guess I got lucky in that one particular area however I dont recall ever putting this up for discussion. I went out and purchased whatever guns I wanted and keep them out of sight. I did ask her to attend classes with me however she flat out refused. Recently she has begun to change her perspective and is relieved I have a chance of protecting our family should there be a need.
I dont know how you address this outside of what the others are suggesting, for me it wasnt an option for discussion. I was raised in a family of gun owners (most of my family is LEO - the poo poo) and the instant I was able to purchase a gun I did.
Perhaps you just tell her its your decision to make, not hers.