Sex is an important component of any relationship. This is to the extent that the data on the matter strongly suggests that one of the leading causes of relationship dissolution is sexual dissatisfaction. Likewise, research shows that relationship satisfaction is highly associated with sexual satisfaction. While sex is not the most important factor in a relationship --communication is-- it is still important enough that it could make or break a relationship.
A common struggle within monogamous relationships when it comes to sexual satisfaction is variety. When you have one partner, it is often found to be difficult to spice things up in the bedroom. This is to the point where it has been commonly reported that men in long-term monogamous relationships experience a dip in libido as the relationship expands in time. Indeed, this also seems to be the case for women.
What gives? Tradition has taught us that sex is for couples only. Is it the case that conventional norms surrounding sexuality could be wrong?
The long story short here is that monogamy has a built-in mechanism within it that makes sexual variety very difficult and it seems to be that sexual variety is something that many people require to feel sexually satisfied. In many ways, monogamy is destined to fail sexually. After all, if you look at the Reddit thread r/deadbedrooms you don’t see many people who are participating in polyamory talking about a lack of sex. You truly only see monogamous couples with this problem.
Hence, the primary sexual benefit of open relationships. Variety.
The whole idea behind open relationships is that you can have other partners. Given what has been stated above about the connection between a lack of variety and sexual dissatisfaction, in theory, open relationships should be able to ameliorate this issue.
And in practice it does. People in open relationships generally report high levels of sexual satisfaction. However, there is one simple caveat: your sexual activity with other partners needs to be communicated with all people involved. Without this communication, "Sexual activity with someone else besides the primary partner, without mutual consent, comfort, or communication can easily be understood as a form of betrayal or cheating," says Forrest Hangen, a graduate researcher at Northeastern University. "And that, understandably, can seriously undermine or jeopardize the relationship."
This is further implicated by the fact that an open relationship creates more room for higher sexual frequency than in a monogamous relationship. It is quite often the case that a monogamous couple will have good sex, but not often enough for at least one of the partners. An open relationship can help relieve this.
Finally, and probably most importantly, open relationships can take a sexual burden off your primary partner. When in a monogamous relationship, there is an implicit expectation of being everything your partner wants you to be sexually. Quite often, this is a very pressure inducing predicament to be put in. This is due to the fact that you cannot be everything your partner wants sexually: no one can be everything that a single person wants. And that is okay. Open relationships can serve to remove this pressure from a relationship. It might be the case that your partner has many sexual desires that you are just unwilling to explore with them. In an ordinary monogamous relationship, this sort of rejection might lead to resentment and sexual dissatisfaction. But in the context of an open relationship, it can be negotiated that such sexual desires can be fulfilled by someone else outside of your primary relationship.
It seems that as long as all parties are communicating and consenting, not only can open relationships be positive sexually, but the data shows that they can be more sexually satisfying than ordinary monogamous relationships, which can help lead to their long-term success.