[kerkeslager]

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.