It always amazes me that couples stay together for years, decades, a lifetime even. Human beings need new stimulation both physically and mentally to stay 'alive' inside.
My Nan and Grandad had been married for 59 and a half years when my Nan passed, and they had known each other since their very early childhood (my Nan was 82 when she died) due to their parents having been friends.
It was the only relationship either of them ever had, and they always seemed happy enough to me.