... does she also look female in that dress? does she also look blond in that dress? would you lie about these things?Oh yes, it's much more ethical to tell your wife she really does looks fat in that dress.
This is a bezerk button for me so I'm-a-gonna rant. I find the whole cultural meme of "do I look fat in this" insulting to absolutely everyone involved, male or female, fat or thin.
First off, fat is an adjective, not a moral judgment. It's only your own preconceptions as either the asker or the answerer that makes it something that you would even consider lying about.
Second, while fat is an adjective, it is not an on/off state. It's a comparative, it's meaningless without a baseline, and different people's baseline ideas of what fat is can vary a good bit, so what's even being asked?
Third, if the person asking the question really cares about what visual impact they make and is trying to get an opinion on which clothes enhance a particular aspect, how is lying more ethical? Clothes do not change the actual body beneath them, but they can change its shape and they can change the way the eye is guided to look over them. I'm trying to avoid saying that some clothes look "better" or "more flattering" because there is not one central opinion/shape of how all people should look. But if you're actively trying to dress to, say, draw attention to your shoulders or breasts and away from your stomach, how are you helped by people intentionally giving you bad feedback?
Fourth, if you are in a relationship, how can you possibly think it's a good thing to set up a cycle of intentional lies? There's this NONSENSE being peddled by sitcoms and advertisers to lie, lie, lie. "Don't tell her she looks fat even if she does. Don't tell her she looks like her mother even if she does. If she says she doesn't want presents, buy her presents anyway. If she says she'd rather have a new computer than a diamond ring, ignore her and buy the diamond ring. She's just afraid of looking too eager." etc, etc. They tell women that they must lie or be viewed as brazen whores, and they tell men that they must engage in some secret code dance or be "exiled to the couch". This is complete bs on both sides and does nothing but enforce bizarre gender stereotypes and contribute to unhappy relationships - which is GOOD for advertisers, unhappy people cycling through multiple relationships will BUY MORE CRAP.
... now, if you're in a relationship with someone who's new to the whole idea of actually being honest and working together instead of engaging in some mars/venus bullshit, you might want to find a TACTFUL way of answering the question. but outright lying? Is counterproductive. It's not just unethical, it's actively shooting yourself in the foot and lowering your own chances of later happiness. Kinda like piracy!