After being married for many years, many women find that they just aren't sexually attracted to their husbands anymore. Some of these women, suffering from both low desire and sexual response difficulties, consider infidelity to determine if the blame lies in their partner or in themselves. More often than not, they are devastated when the problem persists with another partner, or when their partner finds out, or when they disclose it to their partner—just to name a few possibilities.
It is unlikely that having an affair will help you find the answers you seek. If the problem is your relationship or your partner, on some level you know it. You may not be clear on all the pieces of the puzzle, but you do already have some sense in your heart... you just have to look there. It is important, however, to reach out for help —both medically and emotionally. You should think about being evaluated for testosterone levels as well as other medical causes (Remember: testosterone declines slowly throughout the lifecycle).
It would also be useful to talk to a therapist (either alone, but even better with your partner) to determine what relationship or emotional factors are at play. For instance, where should your desire really be after many years of marriage? Is the issue that you are having trouble adjusting to the "attachment" you feel toward your husband instead of the "infatuation" you felt so long ago? Is there a way to readjust your perspective on him and the relationship, which would open you up more sexually? Are there things he could be doing to help? These are just some of the important questions you should be addressing with him in a safe environment with a trained therapist.