I sometimes wonder what people actually talk about before they get married.
I myself got married within 5 months of meeting my wife-to-be. That was 30 years ago. I wouldn't necessarily advise people to get married so quickly, but hey, it worked for us. We are still happily together, and have two wonderful sons to show for our efforts.
Now, polyamory was not in our consciousness at the time, so of course we didn't discuss it as such before we got married. And when you are so happily in the grip of a wonderful (and sudden) love affair, as we were, having sex with others is the farthest thing from your mind - when would you find the time and the energy, for one thing?
Nevertheless, we had talked a lot before tying the knot, and we quickly learned where the "boundaries" were. No smoking, no drugs, no dietary laws, yes children, no surgical procedures on male offspring (obviously, there is some Jewish blood in there!). It's not as if we couldn't imagine saying NO to something. But "infidelity"? I maintained then, and have maintained ever since, that it is not necessarily wrong to have adventures outside marriage, as long as it is not done to humiliate your partner. My wife-to-be smiled and married me anyway.
Now, if someone is so adamantly against infidelity of any kind, she (or he) should say so loud and clear, before the marriage ceremony. Since about 1965 in western countries, it is simply naive to rely on a few words in the marriage ceremony to reinforce an attachment to absolute fidelity. Besides, the same ceremony also contains references to "till death does us apart" and "for better or worse". How many people take those words seriously, what with divorce rates above 50%?
So, ladies and gentlemen, if you want 100% fidelity, say so early and clearly - and keep on saying so throughout the marriage (or other kind of relationship). Assuming you find somebody to marry you...