I think my wife would be offended but I wouldn’t be surprised or offended. My wife is very physically attractive and I tend to expect men to be attracted to her.
I don’t need to think of how people who I know would look naked - I have actual porn. (On the internet, it’s available for free!)
Plus, to be perfectly honest, I prefer not to speculate about something I’m never going to see anyway. I’m more likely to fantasize about random people I don’t know (see: porn) than people who I have definitively crossed off the “possibility” list.
It looks like a double bind to me when somebody deliberately thinks of that question and asks what they as mature adults think is most salacious and then act scandalized when you kindly give them the pleasure confirming their phantasies.
If they act offended I would try to turn the tables and point their own holy virtuous finger at them.
Puerile games you play, BTW. And your “opponents” played in bad faith, IMO.
(I have skipped 4/5ths of this thread, in case I have been ninja’d unbeknowst to me)
I loved that comic! Is it really “decades ago”?? 
Am I that old?
Does anyone remember that TV channel <ahem> decades ago that consisted entirely of videos of women with perfect bodies doing slo-mo yoga poses in scanty yoga togs? Lots of close-ups during the stretches.
Upon reflection, I think the best answer to “Have you ever thought of someone’s in the room’s spouse naked?" is, “Not just the spouses, but everyone in the room. All the time. Stark naked nude. Not a stitch. I have a hard time picturing anyone in clothes, even when I’m looking right at you. I don’t know why, but I can’t help it.”
That’s much funnier than my answer, but I think the best answer (both safe and true) is “well, now that you bring it up, yeah!”
It’s not creepy (I’ve been fantasizing about you for years!) but neither does it imply the person is unattractive (no, why would I have done that?) and as mentioned, it’s almost certainly true that you thought about it as soon as the question was asked.
You could also say, “Give me a minute…” then pause a couple of beats, and then say, “Okay, yeah!”
Winner!
… wrinkle your nose and say “well I won’t be doing that again.”
“I don’t like making guesses about details without evidence…could one of you help me with a visual aid?”
@Asuka Okay, now you need to go back to that group and demand to play that game again. You’ve got a whole range of possible answers to the question. 
Then come back to this thread and report.
As Rumpole once said to a young barrister: “Contempt of court, like blasphemy, is best conducted in the privacy of one’s own mind.”
“Thought of, I’ve had sex with your wife!”
Although some people–probably quite a few–have a great deal of trouble differentiating between sexual nudity and non-sexual nudity.
As far as involuntary thoughts, Martin Luther used this analogy: “You cannot keep birds from flying over your head, but you can keep them from building a nest in your hair.”
But were the lights on?
I came to the thread expecting a debate on definitions of morality, but had a lot more fun with the snarky replies! I’m of the group that would have answered honestly, but passed it off with humor - such as look at my wife, leer, and demand to plead the fifth. 
And I would like to say that in general, if someone is reasonably attractive, I as a male, have probably at one point or another had the image or thought run through my head for a moment - but not a thought I’d return to. So I agree with the OP and disagree with the OP’s host - he’s probably on the ‘normal’ side of the spectrum of human behavoir.
Okay, going back to my earlier assumptions about the thread, using the top M-W definition:
1a : of or relating to principles of right and wrong in behavior
So, I feel safe to judge by deed rather than thought - if you act on it though, well, at least this group of friends seems likely to judge it immoral. However, if they were open relationship / swingers, they could consider it a welcome compliment or invitation.
However, as with all moral judgements, the answer is going to depend on the cultural assumptions of those participating. From my POV informed by years of growing up in the midst of Reform-Jewish-Argue-With-God-and-Win-on-technicalities (heh) I find that thoughts and intent are secondary to action, especially after a minor in psychology [ let’s not get started about how absolute altruism cannot exist in a human ].
But from the POV of some of my extremely Evangelical co-worker, intent and thought are just as much a sin as an action. From their POV - to covet thy neighbor’s wife, even in the mind is one sin. To take action would be a second. So as other posters have mentioned, if this is how they respond to an honest and open answer, the OP’s host has chosen a very strange game for mixed company.
I wasn’t acquainted with that quote. It’s nice.
One of my rules in life is “Never ask a question you don’t want to know the answer to.” While I personally would have deflected with a joke, your friends should not have asked if they were not prepared for the answer. I generally don’t hold thoughts themselves to be immoral. But, I do think certain thoughts are dangerous to entertain. If I constantly fantasize about killing my boss, odds are that I’ll eventually snap and assault them. If I constantly covet my neighbor’s wife, I’ll be unhappy. If I have a partner or spouse, they’ll be unhappy. Sooner or later, my neighbor and his wife will be unhappy too.
You shouldn’t have looked at his wife, half-smiled, and licked your lips before saying “yes”. It is a dead giveaway. 
Morally wrong or not, a friend of mine admitted to imagining everyone at work naked and he was fired. It was nearly impossible for him to find another job teaching kindergarten.
Is it really that common? I don’t routinely think of people I know naked, or have to refrain from doing so. Maybe it’s a man thing.