It depends on how you feel and what situation you're in. Your marriage may not be as romantic as you hoped, and you may see your husband's friend as a new paramour, but other people, family and friends, will you as not a good person whose morals are low. If your cheat with a close friend, at least your husband can make a choice not to have this person as their friend anymore. However, let's say the person you're having an affair with is a relative of your husband? That sibling will always be your husband's sibling.
Family gatherings will be very awkward and tension-stricken. If you find yourself in a loveless marriage, it would be better to end it amicably and begin a new relationship with someone who has no relationship with your husband, by blood, work or school. There's no need for unnecessary drama, if you want genuine happiness.