Frenzied Wombat
Potato del Grande
I have tried throughout the general threads to say that a single life, while easier and arguably more 'fun' is ultimately meaningless in both the religious and non-religious views. From a purely non-theistic viewpoint we are here to sire our progeny with desirable mates and see them live long enough to sire their own with more desirable mates. All data available points to marriage being the best and most efficient way of achieving that goal. It has the side effect of giving your life purpose in the face of all adversity. Finally, your progeny take care of you when you are too old and infirm to do it yourself. This is an oft overlooked fact that is now settling in to the every day lives of my parents and older co-workers.
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You are right about the evolutionary benefit of marriage. You just left out one thing-- all the components that comprise the original definition of marriage no longer exist. Power dynamics, division of labor, fidelity-- you name it-- it's all gone in favor of the Choose your Own Adventure scenario we have now.