Like it says in the Poll. What would your immediate reaction be? Feel free to elaborate.
Poll to follow?
Mighty quick on the draw, there! I think I know your answer already! 
I think I’d freeze because I would be very confused. “Where the hell did he get that from?” would probably be my first thought.
Walk over and take it away from her, and ask her where she wants it stored.
I’d assume I did something irredeemably wrong, that made me deserving of getting shot, and would try to run up and attack them so they were justified in shooting me in self defense.
Freeze in confusion. I know he loves me. We have three guns, and the mere thought of him aiming one of those at me makes me already confused. Why would he do such a thing?
We always said this would end in a murder/suicide. 
Interesting. So far noone trusts their spouse enough to automatically assume there’s some sort of threat behind them that their spouse is aiming at.
Freeze in confusion. She’s very comfortable with me having a weapon or two around the house for security but knows nothing of their operation herself. She wouldn’t point it but would hand it to me instead were there a threat present.
If there was, I would be impressed if he’d managed to get to the guns, unzip them from their case, get the magazines, load them, load the guns, and still have time to aim the freakin thing.
If there was a monster/enemy behind me, he would probably grab the closest thing at hand, not the guns.
I’d guess we’re playing one of our sex games.
I’d probably start laughing, as she has no idea how to operate a gun. If the look on her face was one of fear, I’d look behind me to see who she was pointing it at.
Given his ability to aim I’d drop to the floor in panic. I’m in more danger if there is something behind me than I am if he’s mad at me.
I’d reflexively say, “Give me that thing,” and hold out my hand for the gun. Whenever I’m confronted with something that conflicts with my understanding of the world, I react just like my mother. I would probably sound so authoritative that he’d hand the gun over before he realized what he was doing.
Under no circumstances would I think he was going to shoot me with it.
Look over my shoulder. Mrs. D has been a gun gal for 40 years, so she’s pretty handy with one. Maybe I missed the monster behind me, and she’s spotted it!
We promised each other when we got married that we would never kill each other. That’s just selfish and uncalled for. Things not working out? Walk away, divorce, and be done. It just seems the more reasonable path.
Being married 24 years last month tells me that she can tolerate far more bullshit than I ever thought possible. I recently forwarded her nomination for Sainthood to Rome and she wouldn’t want to jeopardize that.
If someone pointed a gun at you, you would assume you deserve it and help them kill you??? 
I know you deal with self esteem issues but that’s a bit much don’t you think?
I’d freeze. Neither of us own or have any desire to own a firearm, and the mere image of my spouse holding a gun, let alone the fact that she’s aiming it at me, would be such a distortion of my world view that I wouldn’t have any idea what to think.
With my ex I would just think, “Yes, of course this is happening.”
Knock it out of her hand even if that meant she would kill me. Pointing guns at people is wrong. And discourteous.