Opening Up by Tristan Taormino, is a great starting point for reading about how people have successfully navigated this situation.
The single most important thing is communication, painfully honest, absolutely frank, and fully open communication. This takes time and multiple conversations with time in between to absorb and assess what you’ve talked about. Be aware that fantasy and reality are often very different. Ethical non-monogamy requires careful attention to the impact of your choices on others, everyone who might be affected, not just the primary couple.
Thinks hard about what style of open relationship you want. Is it just about sex? Consider swinging. Is it OK to fall in love with an outside partner? That’s polyamory. Is it just about occasional sex with strictly defined limits on partners and acts? That would fall under the umbrella of being monogamish. Be prepared for your or your partners' feelings on this to change.
Some issues you should discuss with your partner include: *STI protection *sleepovers *sex with friends/acquaintances *when to tell each other about potential outside interests *vetoes *specific sex acts you may want to reserve for just the two of you
Other great books include The Ethical Slut, More Than Two (and its great website), and of course the sister subs of /r/nonmonogamy, /r/polyamory and /r/swingers. The two books listed are oriented towards polyamory but have a great deal of good material that applies to all open relationships, covering issues like jealousy, communication, partner care, and more.
One issue that a lot of couples at this stage wrestle with is trying to control emotions. A lot of rules are put in place to protect the primary relationship at all costs. There are two major issues here. First of all, feelings just happen. They are not subject to rational control. The second is that the outside partners are people with feelings of their own. Even in primarily casual sexual relationships, they are not robots. Breaking off a second relationship may be intended to preserve the primary relationship, but it can mean hurting an innocent third party because of your own insecurity and that’s hard to square with ethical non-monogamy.
This may seem like an argument for polyamory: It’s not. It’s an argument to go in with eyes open, be honest when feelings get complicated, and be respectful of your partner’s partners.
*(To be clear, I liked everything you wrote, but like you said, brevity is really important here, so I took out some of the more detailed reflection)