There's nothing weaker than a startup founder who gets furiously jealous if his employee aids and abets another company. It's the confident ones that know they're the best that aren't afraid of competition. No reason to get litigious, a little competition is all.

Jealousy is a basic human emotion of paramount importance. Loyalty is a cornerstone of society. I value deviation from the norm as much as the next free spirit, but deviation from this basic principle I will not allow myself ever. There is nothing insecure or weak about the territorial insticts humans associates with things of value. It may have a perfectly sound evolutionary explanation, it has a perfectly sound religious explanation, no matter how you split it, it's there. I don't buy that it's cultural, except may be some manifestations of it. There must exist a core which transcends culture. Perhaps protection of offspring can form one tenet of such core, but I'm no expert and this is not a proof.

Nonetheless, let me give you an example of how jealousy is simply a construct which one can use to describe a particular response to a given stimulus. Let's say one is walking down the street and notices a pile of trash. One, not prodded or otherwise influenced decides to do the "right thing" and remove the trash. The prospect is not pleasant, but one proceeds out of some motivation that is not relevant. Then let's say someone swoops by and picks up the trash and hurries it away. The first individual may very well be relieved of no longer having to deal with the trash, and feel grateful for another's "brazen" act. Let's replace trash with gold. Sure, some people may say: "meh, it's just a pile of gold, let 'em have it!". Some might get very upset. See, here we have jealousy measure a degree to which a given individual can raise their defenses. It's a protection mechanism more than an act of insecurity. Just as carrying a gun to a gun fight is a protection measure. If you chose to bring a knife "cuz you're confident", you'd get a few in the dome and that'd be the end of it. Just as any old general strategy or planning. Granted, one can still lose even with a bazooka, and some can win with a knife, but one had better be prepared regardless.

So back to relationships. If one partner decides that they're liable to "chat someone up and accidentally have sex" they should divorce/separate and carry on. Unless their significant other has agreed that it's normal and is liable to do the same. Those relationships work, they're fine, but they're a contract of sorts. Let's not conflate the desire to keep something special, with the inability to let people "talk to others". If we define "talking to others" as being a precursor to a steamy side life, then we're talking of different things. No one owes anyone anything, unless one has a previous arrangement which forges a contract. However, when we enter a mutual contract with another party, a modicum of self worth is good to keep honest to oneself, and the loved one. Of course people make mistakes, and if they so choose, they can separate and pick a different life. But back stabbing is one of the most weak and worthless acts of human nature. There are amazing stories of couples that not only went through true horrors in their life and remained together, but remained loyal in the face of seduction from a rather "appetizing proposition". I respect, and dare I say even admire such people. Lastly, since it's inevitable that the "self preservation and self interest uber alles" is an always-nascent thought, I would only say this: if one wants polygamy, then there are plenty of partners who also do; pick N.

Sorry for the rant.

Edit: response to your edit:

> We shouldn't be scared of trying things that aren't best, and if something is genuinely good, it shouldn't fear criticism and rivals. If it's good, it will beat those rivals.

I'm not afraid to admit that I'm not perfect and that I experience full range of emotions when it comes to loved ones. Had it been otherwise, had I thought that I was the best thing that ever graced this planet, perhaps then I would have no motivation for jealousy. Likewise I cannot eradicate it, since I see it as innate. Perhaps some people are just not able to experience jealousy at all. Perhaps. I doubt it, but I cannot know, nor can I avoid it. Nor do I know anyone who can. Nor will I ever deny that I do. Best I can hope for is that I'm good enough for the love of my life and that we can go to the end together. If that doesn't work, it was not to be. I can do what I can to keep the relationship working on all fronts, physical and otherwise, but I cannot pretend that nothing better than me can arise. There are truly lucky people who have loved ones that choose what they have, over the greener pastures. We can't all be perfect. Likewise I'm all for experimentation, outside of marriage or a relationship that is, or in a relationship that is predicated on experimentation. I'm just appalled at an apparent justification of betrayal.