How likely is it for a female to end up sleeping with her guy bestfriend? And isn't a bestfriend someone you just really like on the low?

Can someone tell me their experience with their "best friend"? cause I think these two might as well just date each other if they are that close and inseparable.
How likely is it for a female to end up sleeping with her guy bestfriend? And isn't a bestfriend someone you just really like on the low?
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