Not deflecting from the topic, but I believe the truth is human beings aren't "loyal."
Disregarding the concept man has established as "morality," most human beings will engage and seek to engage in activities that bring them pleasure.
Some of you don't believe it, but if your ex was sexually appealing to your best friend, if there's a chance you wouldn't find out about the encounter, there's a significant chance your boy would sleep with her if it came his way.
Situation reversed, there's a great chance some of you reading this thread would do the same if your boys ex attended your college and lived in the next building over.
If you didn't, you certainly thought about it.
I've been there. Best friends ex wanted to come over and was hinting to us sleeping together throughout the summer.
He lives in Tuscaloosa now. Would have never known if I had decided to break her back.
Humans love with instant gratification. We do things that make us happy. We're selfish.
Even those who remain loyal to their close friends and significant others, and push that temptation to the back of their minds have moments of weakness.
The strength lies in not succumbing.
Loyalty is simply valuing the feelings and trust of your s/o/friend more so than you value your lust for pleasure, superiority, and personal gratification.
That's asking a lot of most humans because contrary what a lot of people want to believe, if we're going by the man made concept of morality and right & wrong, human beings are shytty creatures.
In that regard, I definitely define loyalty as a masculine trait. It take a lot to defer your personal desire for pleasure & superiority for the benefit of another.
It's harsh and maybe even Machiavellian, but I don't expect women I deal with to have unyielding loyalty.
The same way I don't expect my best friend to have turned down my ex if she threw the p*ssy his way. I would hope they do.
If I establish a strong enough bond that I refer to you as my friend or mate, I put a lot more stock into you than the other 99% of people I interact with.
In turn, I do my best to remain loyal and hope that it's reciprocated, but I don't expect it because it's not in our nature to do so.
We are always looking for that "lick" to come up on whether we acknowledge it or not.