I can only speak for myself.
My FWB I've known for 2 years now. I had been stood up on a date so was all dolled up with nowhere to go, so went to the local pub where there was music. I was dancing away and ended up chatting to a guy who kept telling me 'that guy over there won't stop staring at you'. Brazen as you go, I invited him to join us (I was on the vodka and red bull).
Anyway, we shagged as you do, never exchanged numbers and that was that. However my Salsa class was in the same pub on a Tuesday night, so two weeks later I bumped into him.
Somehow we became FWB. I was online dating, so would bring my dates to the pub as it was my local.
He never seemed perturbed but would tease me about men draped all over me whenever I'd be at salsa class. I'm a smoker as is he, so we'd usually meet outside the pub smoking and he'd end up coming home with me.
He made it clear from the very start that he didn't want a relationship. Fine. So if a date went wrong, I'd just text him 'fancy coming over'.
Nothing romantic in our relationship whatsoever. It was literally me wanting a hug/whatever and him wanting the same.
He is not someone who I would depend on as a friend. He knows little bits about me. Mostly though, he'll come over, have a drink and a chat, we'll do the deed and fall asleep. He's convenient for when I've nobody else on the go so to speak. He's never rude, very kind and gentle, but it is what it is and it will never be anything else. He doesn't let me in remotely and I know little about him. Quid pro quo, I don't tell him my deepest darkest secrets.
I'm fond of him and people would never know that we had this 'arrangement' running into us.
It's ok if you're not attached. He never minded me having dates. He never speaks about his own private life. I don't think I'd particularly be concerned if he was dating all the time.
Recently we're more text buddies than anything else, so I haven't seen him for a while. But he usually responds a day or two later to any text.