If your wife asks, "Does this dress make me look fat?" And you think it does, but you tell her, "Of course not, honey!" Is that a sin?
A successful marriage relationship will need a full measure of agape love. Here's how Paul defines it with particularly relevant words bolded:
Love is patient and kind; love does not envy or boast; it is not arrogant or rude. It does not insist on its own way; it is not irritable or resentful; it does not rejoice at wrongdoing, but rejoices with the truth. Love bears all things, believes all things, hopes all things, endures all things.—1st Corinthians 13:4-7 (ESV)
Let's take these in reverse order:
Rejoices with the truth
Presumably, the husband does not think he is telling the truth. By this standard, lying to your wife is not loving.
It is not arrogant or rude
However, if the husband said exactly what he thought, that would be rude (and possibly arrogant):
It makes you look like a pregnant cow.
Honestly, given the choice, it would be more loving to lie to your wive than to be rude.
Love is patient and kind
But there's another way. The patient thing to do is to ask questions or let the wife down easy. I've found that saying things like "I wonder if it would be a good idea to accessorize?" communicates that something is not quite right, without being rude. (I don't know what accessorizing means exactly or how it could help, but it shows my wife that I care.) Another thing that works for me is to spend "too long" appraising the outfit. After a minute of looking and contemplating, my wife will get the hint: something isn't right.
If we are about to go out and I think there's a good chance she will be embarrassed if someone posts a picture of the outfit on Facebook. The kindest thing I can do is let her know:
Dearest, I know you spent the last hour and a half getting ready to go, but I really think you'd look better in some other dress.
Then I will (Lord willing) spend whatever time is needed helping her look her best.
Just as a small counter-argument to the obvious answer, remember that Jesus said the entire law was summed up as this:
30 Love the Lord your God with all your heart and with all your soul and with all your mind and with all your strength.’ 31 The second is this: ‘Love your neighbor as yourself.’ There is no commandment greater than these.”
The question, of course, is whether being a true witness would cause injury.
Oddly enough, if your wife is just looking to see that you love her, I would argue it is loving to "lie". A better tack, however, might be to answer a different question - to say that indeed, to you, she looks beautiful. And really, her words may be "Am I fat?" but she's really wanting to know, "Am I beautiful?" Answer that question instead, and you'll be safe.
If she were asking a medical question, a loving answer might be, "Dear, the doctor does want to you lose weight, and because I love you and want you to be with me for a long time, yes I think you need to do what the doctor says."
If she insists on asking the question out of vanity, however, then may I suggest the best answer is found in Job 40:14:
Behold, I am of small account; what shall I answer you? I lay my hand on my mouth
I don't see the problem here. The honest answer to "Does this dress make me look fat?" is never yes. Tell her "No, It's being fat that makes you look fat." Tell the truth. Then be prepared to outrun her.
In all seriousness, however, my wife would prefer that I tell her when something isn't flattering because she'd rather have me tell her in private before she can go out in public. If I answered "no", and later one of her friends told her the opposite (and one of them would) she'd be mad at me for letting her go out and embarrass herself.
This question is simply a variation of a question I am amazed that people keep getting wrong:
What is more loving? Telling someone the truth (lovingly) to save them from one type of pain, embarrassment, or some other worse fate, or lying to spare their feelings (another type of pain)?
Telling the truth and hurting someone's feelings short-term is not necessarily the un-loving thing to do. What some people perceive as "mean" is really more loving than giving/telling them what they want to hear.
Sometimes it takes a more extreme example of the application of the same principle to clarify the issue, so here's a more extreme example of the same basic question.
If one of your family members is a drug addict, or is drinking themselves to death, it it more loving to confront the problem, or to turn a blind eye, keep your silence, and let them destroy themselves 9and perhaps their marriage, hurt their children, you name it.) They would think you're a jerk for confronting the issue rather than letting themlive their own lives, but keeping silent is not the loving thing to do in that situation.
Back to the whole "does this dress make me look fat" question, that is often used as a straw-man argument against the idea that we are to tell the truth at all times. The simple fact is that a lie is a lie. We lie for different reasons, sometimes to protect ourselves, sometimes out of a desire to avoid unnecessary conflict. It's cowardly.
If you're really serious about the question, tell your wife the truth in a kind and loving manner.
If it were me speaking to my wife, I'd say something like "Honey, I'm biased. To me, you're the most beautiful woman on earth, and would look great even if you were wearing a burlap sack, but the dress isn't very flattering... I think you look much hotter in this one." Or simply "This one looks better".
As a side note, my wife has an extremely good sense of humor, so I can get away with statements like the one in my opening paragraph. (One of the many reasons I love her so much.) And she hates being lied to above all else, so my answer is a bit "flavored" by my own relationship with her. Not all marriages work the same.
I think the real answer here is to know your wife. My wife requests that I be completely honest with her in that kind of matter. She, believe it or not, really wants to know, so she can change into something else if something isn't the most flattering on her.
I know it goes against everything you've been taught on the matter on how to treat your wife, but really, just like the dress, there isn't any one size fits all answer for this one.
I agree with some of the other comments that say that it takes wisdom to know what your wife is really asking, so you can answer the question appropriately, but if you don't know your wife, that's the bigger issue.
Here's what the bible says:
Leviticus 19:11 NIV
"Do not steal." "Do not lie." "Do not deceive one another."
Psalms 31:18 NIV
Let their lying lips be silenced, for with pride and contempt
they speak arrogantly against the righteous.
Proverbs 8:7 NIV
My mouth speaks what is true, for my lips detest wickedness.
Proverbs 26:28 NIV
A lying tongue hates those it hurts, and a flattering mouth works ruin.
and there are a 100 more that I can post. It shows that there is no good enough reason to lie. However, if you're asking about losing your salvation for lying, I'd point to this:
Ephesians 2:8-9 NIV
For it is by grace you have been saved, through faith—and this is not from yourselves, it is the gift of God— not by works, so that no one can boast.
Which means that your works do not get you into nor out of grace.