Look at me, coming in late.
I have been married to the same man for 22 years, since about six weeks after I turned 18. I've *never* been bored with him, not sexually, not personally. I've been angry, frustrated, disappointed, resentful, sad, happy, in love, proud, tolerant, amused...all the normal things for a normal relationship. I've known people who disapproved of our monogamous status, I've known people with an open marriage, I've known people who cheated and people who were polyamorous. I've done my share of judging other people's business, and being judged by others. **edited to add: We were monogamous for 18 years, and our reasons for becoming poly had nothing to do with boredom. In a twist, the person who criticized us most vocally for our monogamy ended his long-term open relationship at around 16 years and is now monogamously married to someone else. I think that just underlines my point about crafting each relationship to fit the people in it, and maybe adds that sometimes you change together and sometimes you change separately.
Relationships are relationships are relationships, and can NOT be defined by anyone but the people who are in them, or you're heading for disaster. You decide what is important to you, you communicate it to your partner(s), and they react according to what's important to them. You show respect for your partner(s) and their desires, and you hope that they will show respect for you and yours.
Open relationships work when the people involved communicate their needs clearly and work to make sure those needs are met.
Polyamorous relationships work when the people involved communicate their needs clearly and work to make sure those needs are met.
Monogamous relationships work when the people involved communicate their needs clearly and work to make sure those needs are met.
Need I go on?
Sometimes, a relationship ceases to meet the needs of everyone involved and must end. This is not a measure of the worth of the people who must end the relationship. It happens all through life. Sometimes people seek reassurance that a relationship will be "forever", but there is no true way to guarantee that, and you're denying yourself joy in the now by spending your time worrying about the future.
I think, in my humble opinion, that you should worry more about the fact that your desires--for an open relationship--and your fiance's desires--for a closed relationship--are not meshing. If the two of you can not agree, then the relationship will not be fulfilling. The two of you have to craft the relationship to fit you, not whoever answered you on Yahoo Answers.
Okay, rant over.