Question #1: What kind of open relationship are you looking for?
The term “open relationship” covers a pretty wide territory, so there are lots of choices. Some couples only engage with outside partners at specifically designated “swingers’” events or in each other’s presence (a threesome is one example). Some follow the traditional European model, where extramarital relationships are permitted as long as they’re not talked about and they don’t interfere with family time. Other couples discuss these things more openly.
Couples who just want sex and nothing else will often try to guard against romantic feelings outside the relationship by instituting rules such as never having sex with the same person twice, or never having sex with someone you know.
As you can imagine, sometimes these rules work … and sometimes they don’t. Rules can easily get broken in open relationships, just like in conventional ones.