When it comes to dating and relationships, there's no one 'right' way to do things.
That's a great way to start this article. People need and want different things, and more importantly, have different things to offer others. One size does not fit all.
Due in part to the sex-positive attitude of many Millennials, people in open or polyamorous relationships have been able to be more outspoken about their romantic lives than they might have been in generations past. Sure, the stigma around non-monogamy has not entirely faded, but curiosity and polite interest in something is much better than outright intolerance in my book.
For sure!
According to a study published in the Journal of Sex and Marital Therapy, one in five Americans has participated in an open relationship of some kind.
Some polyamorous or polygamous relationships are not open. They are ethically nonmonogamous.

The piece then runs into a bit of questionable citing of statistics. Data is analyzed from a dating site catering to ethically nonmonogamous people, citing what percentage of the websites users are in which states. The problem with this is that one would expect the highest percentage of the users to be from California, given that it is the nation's most populous state. The percentage of users residing in any given state would have to be compared to that state's percentage of overall US population to see if there is a significant difference, in order to see if nonmonogamous people are really more likely to be in one place or another.

Strict monogamy, in the sense of someone having one and only one sexual partner or relationship for their entire life, is extremely rare. Not as rare, but still rare (especially when including cheating) are people who stick strictly with serial monogamy. Even people who think of themselves as monogamous or have monogamy as their relationship goal will have have sexual encounters with, for example, an ex girlfriend or boyfriend while starting to regularly date (and have sex with) a new partner, or will be dating and having sex with two people for a while until picking one over the other, and many couples who generally identify as monogamous have experimented with casual threesomes or swapping with another couple or going to a swinging party. Add in polifidelitous relationships found in polyamory and polygamy (including polygyny, polyandry, and other forms), polyamorous relationships in general, open and "monogamish" marriages/relationships, and "lifestyle" swingers and it becomes apparent that ethical nonmonogamy is not rare at all.

Stop pretending monogamy is the only way for people to live (we tip our hat to the asexuals and aromantics and others who also don't live in monogamy, as well) and stop punishing people who live nonmonogamously in an ethical way.