I believe open relationships are gaining in popularity because there are many ways in which they are superior to monogamous ones, for those mature enough to tolerate them. I believe it is unrealistic to expect one person will have everything you need for the rest of your life. The fact that the majority of so-called monogamous couples cheat also points to the fact that as humans, we are the most sexual of all primates, and not necessarily wired for monogamy. Otherwise, why would other partners be so appealing that people risk everything to cheat?

Also, why not face jealousy instead of avoid it? In a way, open relationships force the next level of emotional growth by requiring people to be open, honest, vulnerable, mature, and excellent communicators in their relationships.

It's not for everyone, but so far, I find it better than monogamy ever was for me.

I don’t think you need someone else if one person can do it for you. Not death do us part, but if you’re happy with loving that one person and all, it just raises too many insecurities.
Penguins, dik-dik, cranes, twinkies, etc mate for life.
I just can’t walk into a relationship knowing that the other person is not 100% available to me. And I can’t take from another person knowing there is someone else needing them as well.

It's a way for couples to prolong their relationships past the point where they've lost some interest in each other.
The old joke is that if newlyweds put a penny in the jar each time they make love, then after a year, start taking a penny out of the jar each they did it, the jar would never empty.
This is, sadly, too true.

Inviting others into the sex play keeps couples together since they can have their cake and eat it too, and this has been popular for centuries anyway, before divorce was allowed. Read any 17th-19th century romance novels.

It is also done to satisfy bisexual people, so they can enjoy sex with either gender, while still maintaining a conventional marriage.

The youth must be becoming thick skinned

I didn't notice. how did you?

Many reasons possibly.
I don't regard myself as in a relationship these days hence still looking.
However I like to have someone to go away with for a few days, company on a drive
not to see a movie alone and so forth.
Perhaps open relationship are the next best thing?
I know many women who are not looking for a guy
or who would even decline a relationship or even a date.
I guess I am in the friend zone so they will go out with me.
I don't consider my situation an open relationship though.

I’d say it has to do with so many factors that aren’t even necessarily sexual/love based. How old are millennials now? Mid to late 20s, 30’s. Look how that generation has grown up with “on demand” being an option for everything. We didn’t have that, generation x and older. (I am assuming polyamory is more prevalent based on generational divisions, as I see the subject through FB posts almost exclusively from that demographic) They have had an on demand youth, the smartphone alone has forever changed growing up. Millinnials expect to have unconventional options in their employment, to basically have a settings menu. They expect to earn 6 figures straight out of school. They have entertainment on demand, communication, banking, everything. Why would relationships and sex be any different? If you think now is weird, you ain’t seen nothing yet. We are in the midsts of a paradigm shift, hell, being vocal about your lack of beliefs in a forum like this is part of that shift. These are interesting times, to be sure.

Because marriage is becoming to be viewed as problematic while at the same time fewer people are believing the "happily ever after" fairy tale. Marriage sucks...always has.

I just usually end up as sidebitch.

I couldn't be in an open relationship. I'm not a big fan of marriage but one partner at a time is all I'm interested in.

My guess is because stigma is going away, so people that are into them feel more comfortable coming out of the closet, which then allowed more people to see the concept.

It’s not a crazy concept. Some people want to explore their sexuality without compromising their emotional relationship. Others will cheat or break up, so I can’t judge a couple who understand and accept one another, in spite of social norms.

I'm not sure that they are - they certainly appeared to be more common in the seventies when I was exploring my sexuality.
That said, I have been monogamous with the same partner for many years now - by choice because it works.

Are they?

I realized a long time ago you share life with people you don't get to own them anymore. Me and my female friends have been happier ever since.

I could never be in an open relationship. I guess I'm a little old fashion that way. I still think if you are going to spend the rest of your life with someone at least do something to make it official. Now I'm really not a big fan of children but that's another topic.

I think people are realizing that marriage or commitment is just not necessary in today's society and relationships.

Hangups are apparently being shaken off as religion loses it hold on people and society. Just my $0.02.

I think it's possibly because more and more people are coming to the realization that monogamy is unnatural. So, instead of being labeled as cheaters, more couples are
embracing reality, and removing total physical fidelity from the list of deal-breakers.