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It is easy to confuse Friends with Benefits with Solo Polyamory. After all, either relationship looks the same especially with the absence of commitment.

Many people start out with Friends (FWB) and occasionally enter into a Solo with another person. Sometimes people break off a poly relationship and continue into a FWB.
So, what are the differences?
“unlike many kink relationships, most friend-with-benefits relationships are explicitly sexual and explicitly nonpartnered” – Laura Boyle
“Friends with benefits is a sexual relationship without the assumptions that generally go along with romantic or partnered relationships — it’s a friendship or more casual connection that has a sexual component,” – Carol Queen, Ph.D.

The most familiar FWB situation that many know about is the “Friends” sitcom. The story is about 3 would-be heterosexual couples who live in adjacent apartments. They frequently meet in the apartment where the women live and at a coffee house (“Central Perk”). The plots are generally about who is sleeping with who and the scandal it creates.
Friends was cited in a 2008 study published in the American Journal of Pediatrics for “glamorizing sex while hardly mentioning its downsides, such as pregnancy and sexually transmitted diseases.” But, how much sex have the “friends” actually had?
After thoroughly researching all 236 episodes the answer is 85 sexual partners. Between the six characters of Friends — Ross, Rachel, Joey, Chandler, Monica and Phoebe — have had sex with 85 different people (some of them were the same making the “friends” even closer by making them “wiener cousins,”)
https://tinyurl.com/es3hkacp
FWB’s often arise when people live together (such as college roommates) and have sexual trysts with each other. The couples who pair frequently sometimes get married. Most of the time the members of a FWB go their separate ways.
OhMori · describes 2 group characteristics of FWB’s:
descriptive FWBs (these people like hanging out and fucking each other when it’s convenient), and prescriptive FWBs:
Group 1 (descriptive ) says, wanna hang out and (activity we both like)? Sometimes activity-ing is followed by fucking, sometimes not, either way it’s a good time. Their profile is on OKC and it probably says friends and short term relationships. Or maybe they are an actual friend, or maybe you met them at a meetup or the dog park or a concert or something. Group 2 (prescriptive) says, hi stranger, wanna fuck? Ever done (sexual fantasy)? Look, here’s a picture of my weiner! They’re everywhere. Their profiles everywhere are such that you know very little about their nonsexual life. They probably exist at your office or among some social group you’re in, but you know those are That Guy, they need strangers.
https://tinyurl.com/bd7eu8pd
An anon poster described the difference
FWB means just that. You’re friends who sometimes have sex. It’s not romantic, it’s not going to proceed up any sort of “relationship escalator”. You may be fond of them, or even love them, but it’s not fundamentally a romantic relationship. Polyamory is generally about dating and trying to find ongoing romantic relationships with people. The escalator may look different than in a mono situation due to the fact that you are in relationships with more than one person, but they’re explicitly romantic.
https://tinyurl.com/3sj3e6ea
Many will say that FWB’s lack any emotional or romantic connection that poly ones do. But some FWB’s are just for emotional support.
Solo polyamory has emotional and romantic connections but does not push the relationship escalator.
In “Smart Love”
The biggest danger in a friend with benefits arrangement is that it is often not inherently stable as a relationship structure. It is entirely possible for one person to become more attached to the other than was intended. Sometimes, this is not a problem. If two single people are friends with benefits, and it blossoms into a relationship, there is nothing wrong with that, unless, of course, they were friends with benefits because they knew that they were romantically incompatible.
Often, someone in a relationship will have a friend with benefits. This might occur because the person is quite emotionally satisfied in the main relationship but finds sexual satisfaction with the other person. This is a fine and often very stable situation if both people have other relationships.
If one person is in a relationship, and the other is not, there is a potential for instability. For some people, it is a fantastic situation. For example, imagine someone coming out of a bad relationship who certainly does not want to get into another but desires physical contact. This would not necessarily be a long term relationship, but it could be quite satisfying for all parties.
On the other hand, a single person might get into this kind of arrangement with a person in a relationship, believing that they can handle it, then find their emotions running away with them, causing some discord.
FWB’s may have many relationships without any interest in polyamory. A Solo poly may respect polyamory ethics and requirements but still prefer a singular position.
When a poly relationship breaks up, members may consider a solo poly existence to other poly positions if only to take time to evaluate future arrangements and avoid a rebound relationship . In a rebound situation, a person may avoid commitment altogether as they search for the “perfect” one(s). A solo poly can make romantic and emotional commitments but still retain his singular status.
While building a traditional polyamory relationship, you may consider maintaining solo poly’s to sustain friendships when your poly family isn’t available.