I am heartened by people coming on to say yes, ENM and polyamory are real things. OP asked me what did I mean when I said it "demands personal evolution" - well it's illustrated by the people on this thread saying things like "Who’d be looking for someone who's looking out for the next person, eugh!" or assuming that someone who might want to have non exclusive relationships is automatically sleazy and trying to 'get one over' on their prospective partner or trick a woman into sex without having the intention of emotional commitment or good behaviour.
Those are quite lazy unexamined assumptions about why people connect with one another. Many people simply can't engage with the idea of trying something different, because to consider it brings the scary realisation that maybe they are living an unexamined and unfulfilled life...for no reason. So they try to shut it down or dismiss it as greedy or sleazy.
All of us have an automatic set of assumptions about sex and relationships which bears examination.
I think we all, in relationships or not, should put effort into working out what really bothers us about someone we love being with someone else. For some people, they do this work, examine their beliefs and think ultimately monogamy is best (which in many cases it is). That's great! Then there is a deep choice to keep one thing (sex) between you and one partner, usually a spouse.
But for many other folks, they simply haven't examined what monogamy is and why it is important - even, if it is important. There's a kneejerk spasm of pain when you imagine your DH with someone else. A kneejerk feeling of guilt when you imagine being with someone else and not DH. ENM people have sat with those feelings and worked out where they come from. The Polysecure book mentioned above is a really good example of learning- it helps you work out your attachment style and what exactly you're afraid of when you let go of someone you love.
Personally I enjoy the journey that me and DH have been on - it's not just about having constant kinky sex with everyone (though even that is much easier to do outside of a marriage, usually, see Mating In Captivity). It's about knowing that the level of honesty DH and I have now shared, we could never have dreamed of sharing before. And I feel proud that I have conquered possessiveness and him spending time and feeling love and sexy excitement with others does not leave me feeling abandoned. I know the reason he lives with me and loves me is not just because we "have to" or because that's how society says we have to meet our needs. I know what he wants and I am impressed that he has conquered shame enough to say it to me, and that I am able to conquer shame enough to seek what I want with others too.
I'm also really good now at winnowing out sleazebags, after doing this ENM for 10 years. It's simply uninteresting and unsexy for me now to be with someone who has limited insight into themselves. It's like papering over the cracks and all the sexy trappings and flirting can't hide it. It becomes obvious.
As I said, lots of people haven't done the work and approach ENM in not skilful ways or are juat using it to mean "want quick sex".
But look at everyone monogamous! Are all of them being completely honest and doing work on themselves and their own emotional ability to love? Are all of them going into relationships in completely good faith, sharing any reservations with their partners and being utterly honest about their deepest desires?
I reckon it's probably about the same proportion of sleazebags/people lying to themselves and pretending to be happy when they are not, in ENM, as in the population at large.