Why does men always the first one to walk out of a relationship?

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I wonder why is it men the first one to walk out in a relationship. When it is about third party- they are always the first one to commit infidelity. Are men really like that when they are no longer satisfied with their partner they will just simply dump her and look for someone else? without considering the womens feelings. Some men i guess look at it as a plus factor to their masculinity, but i guess this is being unfair to your partner. what do you think?