1. I cheated on my husband once, on one occasion, before we were even engaged, because I reconnected with a guy I'd had intense sexual chemistry with and we found that chemistry was still there, and even more intense. It didn't progress all the way to intercourse, but it could have. I did try to talk my husband (then boyfriend) into letting me explore a relationship with the other guy; Hubby said no. I came clean about what had happened, he forgave me (and unfortunately blamed the guy, even though it was equal responsibility), and I chose to stop spending time alone with the other guy. I lived monogamously even though it didn't feel right to me and, because of some sexual and social incompatibilities between Hubby and me, I was kind of miserable a lot of the time. About three and a half years after the cheating thing, *Hubby* was the one who suggested that I see other people. At this point, I wouldn't even enter into a relationship where I would have to live monogamously, so the question of whether I would be unhappily monogamous or cheat is moot.
2. I often feel lonely and isolated, but that isn't exclusive to being around other poly people. I've spent pretty much my entire life, from childhood, feeling lonely and isolated, partly due to bullying (which occurred just as much in my 40s as it did when I was a kid), partly due to poor social skills, and partly due to having learned to shut people out because it was better to reject them than give them the chance to reject me. As far as feeling lonely and isolated specific to poly... I actually have *more* of a connection with poly communities than with anyone else, because at this point the vast majority of my social network is made up of people i met through my boyfriend, who has been poly for a long time and so many of the people he knows are also poly. I'm also "out" as poly to everyone who isn't related to my husband, so there's no reason for isolation there.
3. Since until recently I've met all of my partners and prospects on dating sites, they knew before we met in person that I'm poly. The prospects I've met recently have been through FetLife, through a local poly group, or, in one case, in the course of a conversation among several people about dating and polyamory. I probably wouldn't attempt to date anyone who wasn't already part of a poly group or at least a poly-friendly group, and I definitely wouldn't date anyone without letting them know the situation up front.