I was married in my home country, but haven't seen or heard from my wife for years. I'm not even sure how to reach her. We never bothered to do the divorce papers. I came to the U.S. and married a U.S. citizen, and we submitted the adjustment of status paperwork for me. But at the interview, the officer sent us away with a letter saying I needed to provide proof that my first marriage ended in divorce, and she also mentioned something about "marriage fraud." But I love my U.S. wife; this isn't a fraud, I just figured my old marriage was over. What's going on?
If you are already married to another person, then you were not legally allowed to marry your current wife, and so this current marriage is invalid. Applying for U.S. lawful permanent residence (a green card) on the basis of an invalid marriage is, indeed, considered fraudulent. The fact that you meant this to be a real marriage is better than if you were only trying to get a U.S. green card, but it still doesn't save you from a very thorny legal situation.
Committing marriage fraud can make you ineligible for approval of not only this, but any other green car or visa petition you might submit to the U.S. immigration authorities.
If at all possible, your best bet may be to try and arrange a divorce as soon as possible, and then remarry your current wife. Even so, you will have a marriage fraud on your record, so you will need the help of an experienced immigration lawyer to fully analyze your situation help you figure out whether it's still possible to obtain approval for lawful permanent residence.