I'm a fairly new Christian. My wife asked me if I'd thought about cheating on her. I had sadly commited adultery at heart, but I told her I hadn't thought about it. I had lied. I have repented of this, and am on constant guard to keep those thoughts out, but should L tell her the truth? She should know if I would have been honest, however confessing it will only harm our relationship. But telling her truth from the start would have harmed as well. What should I do?
The question your wife asked is one of those "unfair" questions that women seem to love. They are unfair because they won't like whatever answer they get. I sometimes wonder if they are born out of depression and a desire to "prove" they ought to be depressed.
Let's start with a basic truth. Lying is wrong (Revelation 21:8). So whatever answer given has to be true. In this case, you need to go back and tell her that her question took you by surprise and in a fear of saying the wrong thing, you fell back into bad old habits and didn't tell her the truth. Then ask her why is she asking such a thing because you don't want to give her any doubt about your love for her. Finally, tell her that you've have had to deal with temptation. You're a new Christian and are still battling many problem. Because Jesus said, "But I say to you that whoever looks at a woman to lust for her has already committed adultery with her in his heart" (Matthew 5:28), you are working to put that into practice in your life, but there have been slip-ups in the past.
I suspect that when you ask why she asked that it will take the conversation in a different direction and you might not get to what I suggested. But here is what is behind the answer: You give the reason why you had done wrong first -- you are a new Christian learning a new way to live. You affirm that you know it is wrong and why. You emphasize that you are changing for the better.