Back in my 20s, there was a boy I really liked in our friendship group. I kept this to myself - didn't think he'd be interested, and my then best friend had started taking an interest. The two of them had a short-lived affair - she was always keener than he was, but it fizzled out amicably enough.
Several months later, I discovered that said boy liked me after all. We started seeing each other, although I felt a little apprehensive when it came to breaking the news to my friend.
To say I was unprepared for her reaction was an understatement. She - and her sisters, and her mother - waged a hate campaign against me that culminated in them all trying to break into my house one night. In retrospect, I should have got the police involved, but at the time I was just so bloody baffled by it all, they'd only been together for a matter of weeks...! And she had already started seeing someone else!
Yes, I get the notion of 'loyalty', but going out with somebody doesn't mark them as your property forever, especially when we're talking teen/twentysomething relationships, no marriage vows or kids to take into account. That's a ridiculous and frankly unhinged way to think and behave.
After a while, the boy and I split up. But not before I learned that my now former best friend had told him, many months previously, that I just wasn't interested in him 
. In retrospect, I was well shot of the nasty little bitch.
Apols for long post 