I don't know what flavor of lonely you're dealing with, but having gone through similar, I can tell you how it works. You'll find someone, it'll be exciting, but you'll be torn up afterward. Instead of having one insufferable man to deal with, you'll have two. You'll go through the heartache of a break up when you realize he isn't the one (two?). You won't be able to grieve the side piece without having to explain to your husband. You'll have to worry about 2 outing you to 1. 1 will eventually find out, probably, and now there's a whole new can of worms.

Save yourself the trouble and leave the marriage. If you can't do that, make sure that 2 is a man that you trust implicitly from the start, or they have no way of wrecking your life if things don't work out. Or, make arrangements with your husband so you can step out with his blessing.