We have had an open marriage for most of our 20+ yrs together. There is a lot that could be said about polyamory positive and negative, enough to fill up books or at least many long blog articles. It has joys that no other forms of marriage can match but so does monogamy.
Like all relationships, the frequency, content, honesty and quality of communications is the key. Sometimes you run into blind sports where one partner wants to "protect" the other and there is information omitted etc that actually turns into dishonesty. Avoid that. That does not mean you have to be so detailed that it becomes a journal, but in general marriage means you know the most about one another and that includes partners, whereabouts etc.
The other thing is people change over time, sometimes faster than one partner thinks is possible, so make sure you check in with one another and have "staff meetings" about the relationship/family often. Don't assume, assumptions have got me and my partner into trouble more than once.
Currently my spouse has a friend in which they are together infrequently but maintain regular contact. I have a boyfriend that i see 3-4 times per week. This is causing a bit of stress for us, but sometimes that sort of thing flips around to the other partner. It's not a competition. It requires understanding and support for all.
Open marriages have no worse or better statistical survival rate than monogamous ones by most studies, although the data is not extensive and it is hard to estimate based on the wide variety of definitions and variations. Just approach it with the other person's best interests in mind as well as your own.