Stats are just that. Depending which side of the percentage you're on, this isn't necessarily bad news.
I've had several longer relationships, but my 14-year marriage both proved and disproved the stats. After about 4 or 5 years, the decline in sex was mutual. I'm not sure what changed (babies and fatigue, maybe
), but after about two slow years, things became better day by day and, sadly, had been back to multiple times daily for some time by the time we split up.
Sadly, great sex can't save a marriage but it can keep you motivated to try to work it out.