OP, you present a false dichotomy: tell a falsehood or incite strife or hurt feelings. You might try discretion, instead. This requires certain habits that are well worth acquiring.
For instance, if you are angry and do not want to talk about it, instead of saying “nothing” to the person who asks if you are angry, you can say, “don’t worry about it” or “nothing I want to talk about” or “why do you ask” or “I will be fine” or “nothing I can’t take care of by myself” or “nothing I won’t get over” or a hundred other things. IOW, you have the option of putting off discussion of your emotional state without lying about it. You can even talk about what is bothering you while staying on the topic of the problem instead of dragging in your emotions about the problem (!!!).
For instance, when you are asked “are you angry?” and you reply “why do you ask?” and the person says “well, I thought you might hate me because you were fired instead of Sally”, then you can talk about the firing and the justice of it without making the conversation a conversation about your feelings. “Why don’t we forget about how I feel about it, and talk about the firing? I am interested in more information about that, and I do have some questions.” You can hold that your feelings are totally irrelevant to a discussion about the justice of the situation, because they are.
This means you have to reject the notion that other people have some right to wade in whenever they think you have a problem or are curious about your emotional state. You have to be willing to keep your feelings somewhat private when your feelings are not unpleasant, though. If you wear your emotions on your sleeve, OTOH, you’re going to have to take the consequences. When you get upset, it will show, and the jig will be up. Likewise, people who choose to have their feelings hurt because you’re going through this emotion–that is, who think your emotions are ultimately about them instead of about you–or that are choosing their consequences to some extent, too.
When someone tells you “I love you”, you don’t have to bite on the chance that they are fishing for a reciprical emotional reaction, even if they clearly are. For instance, you can reply with regards to your commitment (which is what is really important) and say “I love you” quite honestly when what you would like to do is run anywhere else for the evening. Look at it this way, after all: If you don’t like them at the moment and you’re still hanging around for their benefit, you must love them. It isn’t self-interest that is keeping you around! As long as your purpose is not to give a false impression–saying “I love you” knowing that it will be taken as a commitment when you mean no such thing–you don’t have to answer in the way that is being fished for. If that* is* what they are fishing for, and they insist on cornering you about it, then you may be bound to have an honest conversation about that, and let the chips fall where they may. You may attempt to be coy or to keep your position private, but you may not lie.
As for "I didn’t think telling someone they’re not fat when they ask you was a venial sin…I thought it was being kind. So you mean if my husband asks if I think he’s fat, and he’s not exactly fat but could stand to lose 20 lbs, and I tell him he’s not fat b/c I don’t want to hurt his feelings, that means I’ve sinned??, this is a another question that needs to move to other ground.
If someone asks you, “Does this outfit make me look fat?” they usually aren’t looking for feedback about their weight. They are looking for validation of their looks, because they are feeling insecure about what they’re wearing. It is better to encourage them to be the judge of their own looks. “What does it matter what I think? How does that outfit make *you *feel? If you don’t think it is flattering, you’re not going to enjoy wearing it. If you like it, to heck with what anyone else thinks, including me.” Yes, I’m saying that it is a good policy to always let comments about other people’s looks, if you want to make any, originate with you. If someone else is fishing, always steer them towards validating themselves.
Note that I said always. If you validate others whenever you like their look (or whatever other compliment they are fishing for) and only withhold your opinion when you don’t feel you can validate their desire to be praised, you have made it impossible to withhold your opinion. If, OTOH, you offer opinions when you see fit but withhold opinions when your opinion is soliticited, you have more room to keep unpleasant truths to yourself.
If someone asks you, “Do you think my weight is healthy?” or “Do you think I need to be more fit?” then if you are no expert you can counter that you are the wrong person to ask and then encourage them to seek the advice of someone who can give them good advice. If you do feel you need to offer your opinion, true honesty would bind you to qualify your opinion as the opinion of a layperson, and allow that a real expert might overrule you.
The key is to refrain from making your opinion all that important to other people. Rather, encourage them to decide their own life for themselves. Require them to respect your emotional boundaries, and keep those boundaries in place for the day when you will want them.
A habit of excessive frankness will expose you to exposure whenever you hold uncharitable or self-important opinions. If we’re know we’re going to be uncharitable in our hearts (which is a sin before we ever open our mouths), IMHO we ought to at least live so that our interior uncharity, self-importance, and blindness does the least amount of damage possible. A habit of discretion makes that far easier.
Isn’t there some way you could tell him how much you love him and also encourage him to lose those 20 pounds for the sake of his health.