If you are married to a good guy who is a great father why would you want someone else?

Probably because you want a husband, not a father, and from your question I assume he's not so great in that category. You should work on improving this relationship rather than looking outside for whatever is missing.

He maybe a good guy, a great father, but obviously he is missing something when it comes to you. Is he emotionally there for you? Physically there for you?

Most affairs, emotional or physical start because we are not getting something from our boyfriends/girlfriends or spouses. Many time we take for granted what we have. Its so easy to give a coworker, a friend, neighbor, our attention, time, because we dont want to be rude or seem uncaring. Like the saying says, "you hurt the ones you love". We sometimes give strangers, friends more of our time than we do our loved ones. I dont understand it either, but unfortuately we do.

If you value your marriage, seek a counselor. Obviously you are not getting everything you need from your husband.

Good Luck