Is it inevitable? I don't think so.
However there are a lot of factors that come into play such as menopause, home/family dynamics, and weight gain.
When a woman doesn't feel sexy she doesn't want to have sex.
However with all that being said once a marriage or relationship comes to an end the whole cycle repeats itself with a new person.
Men are just as guilty of this as well.
Everyone goes back to flirting, using sexual innuendo, hitting the gym a few times a week, being romantically spontaneous...etc
I once wrote a hub titled. "Relationships: Do We Save Our Best For The Beginning"
I suppose it's human nature to treat "new things" better than old things. To work harder to impress someone new...etc
Familarity may not breed contempt but one thing is for certain when we first start dating someone new we rarely use the word NO. A couple of years later it rolls off the tongue with ease! LOL! We'd all do better to remember in any relationship we are either nurturing it or neglecting it. When someone feels neglected they start to consider exploring "other options". One has to be diligent about keeping passion alive.
"Monogamy becomes boring when couples become lazy."
"It's easier to maintain a fire than it is to reignite a spark!"