For a long time, monogamy and marriage were considered inseparable (at least in most modern Western societies).

But a growing body of research shows that couples are starting to consider that monogamy may not be for everyone — and in some marriages, partners are starting to actively discuss whether or not they want to be sexually exclusive.

Of course, our understanding of marriage is constantly evolving: These days there's a much larger emphasis on equality between spouses and mutual emotional and sexual satisfaction than in the past, for example.

And now research published in the Journal of Marriage and Family suggests that another big change is happening in how people think about marriage: Some no longer consider monogamy an absolute essential.

That's the conclusion of a study in which a team of psychologists interviewed 90 Canadians and asked them questions about monogamy and marriage. The group included 26 heterosexual females, 21 heterosexual males, 21 gay males, and 22 lesbians. A few interviewees were divorced, but most were a few years into their first marriage.

The sample is small and specific, so we can't use it to draw any conclusions about the population at large. Americans or people in decades-long marriages might feel differently, for example.

Still, the study is alluring: It builds on earlier research and provides interesting insights about how some people are starting to think differently about these issues.

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The first questions the couples answered revealed that people are becoming more open to the idea of non-monogamous marriages. Less than half of all the heterosexual female respondents, about one-third of the heterosexual male respondents, and "relatively few" homosexual couples felt that marriage and monogamy were inseparable, the researchers concluded.

Most people interviewed thought that monogamy isn't something that a marriage necessarily requires. As one participant explained:

I’ll say that it’s different for everyone ... and you have to find what works for you ... [maybe] you’re committed to each other and you’re married but then you guys decide every Friday night we’re going to swinger parties and that’s what we want to do, and that excitement is what brings us together, then awesome. But is it going to be for me? No. Am I going to say, you can’t do it? No.

That "is it going to be for me? No" is telling: Even though a majority of couples didn't think monogamy was essential to a marriage, the vast majority of heterosexual women and men still considered it the default setting in their own marriages. Many assumed that getting married implied monogamy, and didn't think it was something that needed active discussion.

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Same-sex couples, on the other hand, were more likely to discuss whether or not they wanted to be sexually exclusive. The researchers think this might be connected to how same-sex couples were treated in the past. Since they were excluded from marriage for so long, past research suggests these couples may have an easier time establishing relationships that don't automatically conform to traditional rules.

That might be part of why some same-sex couples are more open to discussing non-monogamy throughout the course of the marriage, as one respondent explained:

Neither of us wants to deprive the other of something that’s important to them, because knowing that we were both in this for the long haul, we didn’t want to say this is the rule, you know what I mean? And if somebody felt at some point that they had some need ... we would be open to discussing and trying to figure out how to make that happen for that person.

While it wasn't apparent in this study, other research suggests that more and more heterosexual couples are heading in this direction, too, even though non-monogamous relationships are still somewhat stigmatized, researchers have found.

In their paper, the psychologists pointed to a previous study that found "heterosexual couples were more likely in 2000 than in 1975 to have an explicit discussion of monogamous practices in order to ensure both couples understood the agreement."

It's unclear exactly how many couples are in consensual non-monogamous relationships or marriages (as opposed to ones that involve infidelity, where extramarital relationships happen but are breaking the rules). One study found that somewhere between 4 and 5% of people in the US are currently in consensual, non-monogamous relationships, but estimates vary widely.

While measuring the popularity of non-monogamous marriages is difficult, it does seem like people are becoming more open-minded about the concept — something in line with what researchers have found so far about the relative benefits of these relationships.

"Thus far, empirical evidence does not support the hypothesis that monogamy is superior to consensual non-monogamy ... Because monogamy is a central foundation of our culture, this is rather perplexing," a team of psychologists noted in 2013, arguing that much more research is needed. "It is curious that an institution that is so clearly accepted — even exalted — cannot be easily empirically supported as more beneficial than alternatives."

It's no wonder that some of those alternatives are starting to gain a foothold in the way people think about and understand this core social institution.