My best friend and I both left our respective homes shortly after graduating from high school. We set up a bachelor pad apartment and it wasn't more than a month or so when we started sharing girlfriends, (we would ask them if they were interested-surprisingly many, were), it was the late 60's, the sexual revolution was in high gear. Some liked spit-roasted, others wanted us to take turns. The interesting thing was when each of us got engaged to and married our respective wives, we hadn't shared either one of them.
The 2 men that became my wife's 2nd husbands were not best friends when each came into the relationship, but both men and I did develop "best friend" friendships. We enjoyed various sports, football, baseball, golf, bowling, card games etc. I think the one thing that I really appreciated about both men was how respectful they were to her, even though both would "fuck her brains out" when they had her alone, they both always treated her like a lady. They each bought her nice things, took her out for dinner and an evening out, to a show or dancing. Each took her for a week or two vacations, or on long weekends. Neither man was ever disrespectful to me or her the entire time that they shared her. In my opinion, both of these men were "best friends" to me. GTR