Well, I'm suggesting a complex spectrum here.
1) True monogamists who show no real interest in anyone but their chosen mate. These people can exhibit the "pitter patter" love response years after a relationship begins and show no real interest in anyone else.
2) People in the middle leaning towards monogamy, who may exhibit many monogamist tendencies but also some polyamorous tendencies. These people likely practice monogamy anyway, because they're "supposed to" or because they agree to a figurative cease fire with their significant other. They will typically remain monogamous but may stray under certain circumstances.
3) People in the middle who lean towards polyamory, but who may exhibit some monogamist tendencies too. I think RDreamer sounds like he might roughly fit in to this category. It doesn't mean he doesn't love his wife, it just means he might also be attracted to other people and/or separate sex from love entirely. Jealousy is less of a concern for this group than for group 2, and these people are more likely to accept that this is just how love works for them.
4) Complete and total polyamory, where people are effectively incapable of a long term relationship because they rapidly lose interest in any partner they have. Even then, some of these people might accept the superficial trappings of marriage, but do so only because it's culturally expected of them. Most people who have these inclinations will cheat on their mates as soon and as frequently as possible.
I suspect that people are scattered across the spectrum in significant percentages. Human sexuality tends to be a broadly varying thing (e.g. from completely homosexual to completely heterosexual to everything in between).