Almost everyone who discovers this desire in themselves goes through the same lonely thought: I must be the only one. You are not. The isolation is real, but it is a product of cultural silence, not of the desire being rare.
Why it feels so solitary
Female desire in general is under-discussed, and a desire this specific gets erased almost completely. Because the word sounds like the male term that carries humiliation, people assume the emotional content is identical, and because no one talks about it, everyone who feels it assumes they are alone with it. None of that is evidence about how common it actually is. It is evidence about how badly our culture handles the subject.
What we can honestly say
We will not invent statistics. But two things are safe to say. Writing on sexual fantasy consistently finds that fantasies involving a partner with someone else are among the more commonly reported themes, for people of every gender. And the online communities built around this specific dynamic are large and active enough that no one who joins them comes away still believing they are the only one. Taken together, the honest conclusion is simple: you have far more company than it feels like.
Letting the shame be cultural weather
The shame most people feel about this is learned, not intrinsic. It comes from narrow cultural scripts about what women should want and about ownership in relationships. It does not come from anything actually wrong with the desire. Once you can see the shame as weather passing through rather than accurate information about you, it loses much of its grip. You are allowed to want what you want, and to explore it or not, without the background hum of shame the culture tried to install.
For more on the inner side of this, see what a cuckquean actually is.