I kind of stumbled in to the sharing busyness right away, at the age of 18 when I was going steady with my first "real" GF.
She was a slut, couldn't say no to any man that asked her about fucking her I later found out, and the first to ask her was my best friend. He fucked her when I had passed out drunk at a party (like most best friends do in some or another occasion.)
Today I know that I actually like that my better half's fucking other guys, but back then I didn't, but somehow I didn't break up with her over what she and my best friend had done, instead I let her fuck him again.
So it happens, some month later, I found her again in a similar scenario, I had passed out this time too, and she was fucking another guy. But this time I woke up again and found her in a closet in the basement with a hairy ass between her legs and the sight of her laying there moaning in pleasure with this guy (a guy I hardly knew) pumping his cock inside my girlfriend until he came inside her really turned me on!
I just knew I had to see more of it and you can bet I did too! She fucked just about every guy I knew back then and then some, she just couldn't say no!
So when I finally broke up with her about three years later she literally was my male friends fuck buddy, always up and ready when one of them wanted to fuck her, but the guy who fucked her most was my best friend, her first lover.
And as it turns out my best friend have fucked all my girlfriends AND my two wife's and still fucking my wife today from time to time. Of course I had fucked some of his girlfriends over the years too, but never when I had a girlfriend (Except for the time we had a foursome with him and his girlfriend for a two years period). So today my wife has two lovers, my best friend, whom I seems stuck with, and an older guy we met from time to time the last year or so.
And I like it, I really do! And I'm glad that my best friend "showed" me that sharing was a good thing to do. And glad he still likes to show me this.. he he
FUCK MY WIFE!