My husband and I might be in the "first few years" category, since we opened in spring 2013. I don't foresee the marriage ending because of poly--then again, who does?--but I'm aware we're an exception rather than the rule.
Some of the things that I think helps us work in a poly situation:
1. It was my husband's idea--but I'm the poly one. We are very different people in a lot of ways, and he was unable or unwilling to meet some of my needs. His suggestion was that I find other people to meet those needs, while we stayed married. So he didn't suggest opening because *he* wanted to find someone or had found someone, and I didn't suggest it at all.
2. Despite the incompatibilities we ran into, we are compatible in a number of ways, including (for the most part) sexually. If anything, the sexual aspect of our relationship has improved since poly started, because through exploring with other partners, I've learned more about what I want and have become more comfortable expressing it, and he's become more willing to compromise instead of doing "paint by numbers sex."
3. We've always communicated fairly well anyway, and poly has improved our communication skills because we have more to communicate about, and I've learned new skills and ways of communicating from my other partners, which I've put into practice in communicating with Hubby.
4. He believes in full autonomy. While he believes it's important for a married couple to keep each other informed of what's going on, he also believes in the philosophy of "If it harms none, do what you will." He also believes it's wrong to prevent someone from being who they innately are, and has told me that he will never ask me to stop having other relationships, because to him it's part of who I am.
5. He's a very self-contained person. One of the incompatibilities we still have is that he prefers spending his free time alone and is content just knowing I exist, whereas I need actual interaction with partners, whether that's going on dates or eating dinner together or whatever. While his tendency not to do any of those things with me is still sometimes a source of conflict, it also means that it doesn't bother him to have me off spending the night with someone else, because he still knows I exist and knows I'll be home at some point, and for him that's sufficient.
6. He's very secure. He doesn't (at least that he admits) experience insecurity or jealousy about my other relationships, because he's secure in who he is and in *our* relationship. Going along with that, he's also hyper-analytical; I sometimes call him Mr. Spock because he approaches everything from a rational, well-thought-out perspective and rarely if ever lets emotion have any input in his thoughts and decisions. And from a rational perspective, he recognizes that my other partners aren't/haven't been a threat to our marriage, so to him that means it wouldn't make sense to feel insecure or jealous about them.
7. I don't hide anything from him, unless it falls under the boundary we've established for "too much information" or unless it's something that would violate someone else's privacy. I tell all of my partners what my usual amount of info-sharing is, and ask them to assume that I'll tell my husband most things unless they specifically ask me not to, so there are some things they ask me not to share and some things that common sense says to keep to myself, for example if it involves one of my metamours as well as my partner.
8. I respect and observe his boundaries. There are really only three things he's asked of me in all of this. I consider all of them completely reasonable, and two are things I wouldn't do even if he hadn't set them as boundaries.
There are probably other factors, but those are the ones I can think of right now that I believe are why Hubby and I have successfully (so far, at least) opened a monogamous marriage, and are successfully operating in a poly-mono model.