Why do women monkey branch. Start looking for another realtionship before ending the one that there in?

There was a survey that said 50% of women have a plan B guy just in case it don't work out with guy A for whatever reason. I know a lot of women that end a realtionship and within a week start dating plan B and become facebook offically after 2 months?
Why do women monkey branch. Start looking for another realtionship before ending the one that there in?
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