Hmm. Here's the other side of the coin, I guess:
In my marriage, I'm the one who admitted to poly thoughts & curiosity; it was a no-go for RugbyMan, and after a lot of painful discussion, I put the potential to explore poly aside for him. Sometimes I tell myself we might be able to explore it down the road, other times I recognize how unlikely that is. Fact: this has NOT changed my nature or interest in poly, it just means it's left suppressed and not acted on. For what it's worth, I'm still sometimes hurt that he wouldn't hear me out or acknowledge my curiosity, I'm still sometimes angry that the price of keeping my family and children and love intact is giving up an entire side of myself that I'll never get to know, and I still sometimes mourn the loss of relationships that might have been and aren't and cannot be. He thinks we're doing great, because we don't talk about poly anymore.
People always say about "surprise" non-monogamy that it isn't what the mono partner signed up for. But you know, a lot of things about my life aren't what I signed up for -- we've rolled with career changes, shifts in financial expectations, a move to the suburbs, a mental health issue... and there's pretty much nothing left of the fit, active, confident, downtown-big-city accountant I married. But I don't cry foul about "surprise" health issues or "surprise" changes in career and prospects, because that's... life. Sometimes life deals out surprises. Funny how fault and blame and charges of unfairness are laid when it involves sex and emotions...