Look. Some people are able to detach sex from love and some aren't. For many people the act of having sex with someone they do not care about leaves them generally feeling empty inside.
A fwb situation is usually when you have two people in a very close and fairly intimate relationship. They might be former lovers, they might just be friends who have sexual needs and finds each other relatively safe as a person to fulfill that.
That's what FWB means to me. I think most people confuse FWB with Booty Call. Which is basically when someone calls you up at 1 am and says, "Hey, why don't you come over and bring some beer?" You show up, you ****, you leave. You don't sleep over (unless you pass out because you're too drunk), you don't keep a toothbrush there. You're there for the sex, and then you're gone. That's it, no strings attached. If you don't hear from the person for 2 months, it's no big deal.
"Well, then," the Cat went on, "you see a dog growls when it's angry, and wags its tail when it's pleased. Now I growl when I'm pleased, and wag my tail when I'm angry. Therefore I'm mad."