I have brought up the idea of an open marriage before, as we know many couples who practice it, but his immediate reaction was "NO!" because that is what he thought I wanted to hear; after all, my reaction to finding out about his other extra-marital relationships had been rather explosive. Though the more I have looked into it, poly is what seems would be the fit for us, as it follows what he had already been doing, and I am now interested in pursuing as well.
Each having a separate relationship is what I had in mind, I don't think us sharing another person would be what we wanted, it certainly isn't what I want. Knowing the other is being fulfilled by someone else is one thing, SEEING it would be quite another...