I have friends who got married into monogamy, eventually decided it wasn't right for them, and had an ongoing conversation about an open relationship. The were honest with each other the whole time, and when she got pregnant, they both decided the open relationship thing wouldn't work anymore, and they stopped. I can see them having the conversation again when the kids have grown, and the two of them enjoying an open relationship again.
Nolite te bastardes carborundorum.