If men are the gatekeepers of commitment, how can a marriage proposal ever be declined?

If men are the gatekeepers of commitment then it means he has already analyzed it all and taken the major step. It means he was positive sure it was the right timing and the woman would obviously say yes.

How come even today there are still certain proposals that have been rejected? Why aren't all accepted? What happened there? Why would a loving girlfriend say no to him? He's opening the gate and she's saying no to that.
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True story: my mother's friend declined her 1st boyfriend's marriage proposal. After 3 years you would think she would say yes... but it was a no. She dated around a bit and accepted her 4th boyfriend's proposal. I'm confused on how that can ever happened?
If men are the gatekeepers of commitment, how can a marriage proposal ever be declined?
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