Warning: Do not try this advice when your wife asks if the pants make her butt look fat. The benefits of lying have been proven over and over again in this scenario.
That's simply not true in my experience. I suspect you haven't actually tried what you're saying over any significant period of time to make the claim you're making.
You're lying to avoid a conflict which is simply not that large in the grand scheme of a lifelong relationship. If I think pants make her butt look fat, that's not necessarily even a problem: it doesn't mean I'm not attracted to her in those pants, or that you even need to be attracted to her in any specific pants, or that she should even base her clothing choices on your opinions. And in a more general sense, why is she asking questions she doesn't want an answer to? If your relationship can't handle communicating honestly about very minor things like this, you're totally screwed when it comes to real issues, like the changing nature of attraction as you age, or asking for what you need to feel fulfilled in a relationship, or concern for the person's health at their weight. If you can't communicate in a really insignificant situation like this, how are you going to communicate when there's anything of actual significance?
Honesty with kindness is a skill, and it's certainly not trivial, but lying isn't the kinder option, even in this case.
Coming at it from the other side of things: when someone lies to me to spare my feelings, there's a lot of times where I know they're lying, and that means I can't trust them to give me honest feedback when I really don't know. If I can't trust someone to tell me something negative, then I can't trust them when they tell me something positive either.
I'm honest much earlier in relationships about much more significant things, so it's highly unlikely that I'll end up married to someone who couldn't handle honest in this situation.
Well, that's gonna be complicated. The strategies are going to vary by situation. Honest communication is just a hard part of being a human. But it's worth the effort.
For example, with the "yes those pants make your butt look fat" conversation, that's an opportunity to set a boundary that improves your relationship: "Please don't ask me questions you don't want an answer to." Your relationship doesn't have to be a minefield, where she's quizzing you and any wrong answer could turn into a fight--that's not a pattern that's fun for either of you. Your wife probably just wants you to compliment her, so you can tell her that you'll make an effort to verbalize when you like how she looks or something she does. Those compliments will hold more weight, because she'll know that what you're saying is true. A compliment from someone who gives false compliments all the time is meaningless.
Some other general strategies:
1. Consider that I might be wrong, and if I was wrong, apologize. A side effect of being more honest with others is that I end up being more honest with myself as well, and I've often discovered that I believe some wrong things. Particularly with opinions, if you find that being honest about your opinions is offensive to people, consider that your opinions might be the problem, not the honesty. A lot of people who say "I'm just being honest!" when they offend people are actually just assholes. Lying wouldn't make them less assholes, it would just make them secret assholes.
2. Consider that my input wasn't wanted, and if I did that, apologize. A lot of the time the truth can just exist without being said. Does it need to be said? Does it need to be said now? Do I need to be the one to to say it? I'm particularly bad at this, personally.
3. Have a conversation with them about why I said what I said. A lot of times, like with the "yes those pants make your butt look fat" conversation, what the person is actually taking offense to is because they're assuming that what you said has other meaning. Just because I said the pants make her butt look fat doesn't mean that I don't love her, or that I'm not attracted to her, or that I think her butt looking fat is a bad thing. It may be that her negative reaction is because she's assumed one of those things, so clarifying would make her feel more secure.
4. This doesn't trivially apply to you wife, but in some cases, you can just stop wasting energy on the person. You can't please everyone, so don't try. If someone wants you to lie to them, that means they don't value what you have to say. Why would you want to talk to someone like that? But really, it rarely gets this far, because the reality is that most people don't take offense to the truth. Most people realize they have no choice but to accept reality when presented with it.
You're lying to avoid a conflict which is simply not that large in the grand scheme of a lifelong relationship. If I think pants make her butt look fat, that's not necessarily even a problem: it doesn't mean I'm not attracted to her in those pants, or that you even need to be attracted to her in any specific pants, or that she should even base her clothing choices on your opinions. And in a more general sense, why is she asking questions she doesn't want an answer to? If your relationship can't handle communicating honestly about very minor things like this, you're totally screwed when it comes to real issues, like the changing nature of attraction as you age, or asking for what you need to feel fulfilled in a relationship, or concern for the person's health at their weight. If you can't communicate in a really insignificant situation like this, how are you going to communicate when there's anything of actual significance?
Honesty with kindness is a skill, and it's certainly not trivial, but lying isn't the kinder option, even in this case.
Coming at it from the other side of things: when someone lies to me to spare my feelings, there's a lot of times where I know they're lying, and that means I can't trust them to give me honest feedback when I really don't know. If I can't trust someone to tell me something negative, then I can't trust them when they tell me something positive either.