Cuckolding is actually a completely different fetish. Which is cool, if that's what you're into.
The problem with the term "open marriage" is it is a blanket term applied to many different scenarios. Successful open marriages consist of two people who are polyamorous and have no resentment towards the other person having other relationships. Unsuccessful open marriages consist of previously closed marriages that have opened up due to one or both parties being bored, issuing an ultimatum, a final way to try and save the marriage, etc.
Also, different open marriages have different structures. In our marriage, for instance, we both do NSA play with friends/other couples, but we don't have other "boyfriends" or "girlfriends" because we want to save our deepest emotional intimacy just for each other. Other marriages people have both the wife and the "girlfriend" over for Thanksgiving dinner. That wouldn't work for us, and our open marriage wouldn't work for others.
Different strokes for different folks. An open marriage is just as valid as a closed one. The only thing that separates a "good" marriage from a "bad" marriage is whether or not both parties are on the same page with what they want the structure of their marriage to be, and where they want to go going foward. If you're not on the same page you're headed either for divorce or a long, unfulfilling marriage.