Why is this the social norm? Why do the swingers, the polygamists/polyandrous, the polyamorous, and the down right promiscuous have such a bad light shown on them?
There's this impression that if your significant other wants to sleep someone else that they don't love you, or they don't find you interesting, and you may start to become so overly self-conscious that it consumes not just your relationship, but your entire life.
My idea of an open relationship isn't about being able to sleep with other people aside from your partner. My idea of an open relationship is openly communicating, openly trusting, and openly being honest.
I suppose insecurity is the main factor of why couples don't actually have this type of conversation. The conversation that once in a while they'll bring another party into the bedroom. Or maybe switch partners for a night with another couple they're friends with.
The reason why people cheat is because monogamy is monotonous. We need variety in our lives. Our diets consist of variety, our cars consist of variety, our wardrobes consist of variety. Why aren't more people open to having a variety in their sex lives!?
Just because you want to sleep with someone else, doesn't mean you don't love your partner. Just because your partner wants to sleep with someone else doesn't mean they don't love you. I personally think it helps build a stronger foundation because both parties are instilling trust in each other. Your partner expresses to you that they want to sleep with someone else. Have the conversation of how to approach it best. Wait it out. If the urge is still there within a week, 2 weeks, a month, whatever, decide what to do. Invite them to join you or maybe you've also had the urge to sleep with someone else too. You both decide it's best to switch it up for the night. Whatever the situation may be, at least have a fucking conversation about it. Be open to trying something different.
Your comfort zone is your failure zone.
Personally, I'd prefer sexual monogamy in a relationship once it progressed to engagement/marriage. Prior to that, a little mutual variety once in a while is acceptable.
Have the conversation with your partner. If you both decide it's not something you'd do, great. At least you shared an open and honest conversation. I understand this approach may not be for everyone, for various reasons, but don't let insecurity & jealousy factor in to your reasoning.