Really, just a 'few'!! Tell that to the other 4 couples (friends) that celebrated our 50th together this last year - happily, I might add!!
Our ds & wife have passed the 25th, 2 brothers and wives 50th - also happily, and many others I know. I know of only a few that ended in divorce.
You don't reach these years happily married by having an 'open' marriage -