It's really scientific. Men do actually need their privacy. Literally actually.
Women need to talk to people to release stress. They produce a hormone called oxytocin and the more they produce, the better they feel. Talking to someone who listens helps to produce this hormone, in effect helping them feel less stressed.
Men on the other hand, need to replenish their testosterone levels by relaxing. And by feeliong appreciated and needed by their partner. But they need the down time to themselves to replenish the testosterone they lose during the work day. It helps them be less stressed out.
I do not believe that anyone is simply not cut out for marriage. But marriage is work. A lot of people think it will be easy living with someone else, but it isn't. Marriage is like many things...it gets better with time.