C: “I’m a newlywed and while I love my husband with all of my heart, I’m just not sexually satisfied with him and so I can’t help but to think about other men I’ve been with when we’re…engaged. Is that a bad thing and if so, will it do more harm than good to tell him about what I’ve been thinking about?”

A: Let’s break this down bit by bit. You say you love your husband with all your heart but you’re not sexually satisfied with his performance in the marital bed. My question to you is what efforts have you made in order to communicate effectively your frustrations? What efforts have you made to show your husband what it is that pleases you sexually? What have you done to make the situation better from a spiritual standpoint within to strengthen your marriage?
You say you love your husband with all your heart but you can’t help but to think about other men you’ve been with when you are engaged in the act. How can you spiritually/physically connect with your husband if you’re thinking about previous sexual experiences while you’re making love to your husband?

I want to make it clear that I’m not asking these questions from a place of condemnation. I asked these questions so that you can begin to work yourself through the process of understanding why your behavior is unacceptable as a wife.
To answer your question, yes it is a bad thing that you been thinking about other men while engaged sexually with your husband with whom you’re not satisfied.  I get that you’re a human being with wants and desires. There nothing wrong with wanting fulfillment.  The troubling thing is that your husband is not aware that he is not (at least physically) meeting your expectations.
The underlying major issue that you have yet to address, is the fact that you are not communicating at all about your needs.  I don’t know what your motivation is when it comes to your sexual selfishness (and that’s what it is because you’re having sex in your mind with someone else-Matthew 5:28) but your husband deserves the opportunity to try rectify problem that he did not even know existed!  Your husband deserves the woman who promised to love, honor and cherish him. You deserve to be happy in every aspect of your marriage. If only one person is unhappy in the marriage because that person did not communicate their displeasure, disasters are bound to happen.
Ask God to give you the words and the courage that you seem to lack dealing with the issues you have with your husband and yourself head on. He will strengthen you and guide you on what will definitely be one of the toughest journeys of your life. I’m praying and rooting for you both.