"85% of internet statistics are made up." -- Abraham Lincoln*
TL;DR -- statistics regarding affairs and/or infidelity may be inaccurate.
People have a tendency to seek out statistics for all kinds of information that applies to them ... oftentimes searching for and interpreting results which reflect their point of view (confirmation bias). Those of us in affairs may very well seek out such information, usually to get an idea of how the affair may end and/or to discover similarities with what the "norm" may be for various factors in one's own affair.
Confirmation bias also applies to the
source of information you may be reading on the internet: I Googled for some statistics to include in this post only to discover the website they were posted on was selling services for "private detectives". As I couldn't find a source cited for the statistics listed, they could easily be in the 85% of made up statistics Abe Lincoln* describes, but could just as easily be accurate, or biased in order to sell private detective services to betrayed spouses.
(I'd also stumbled on older Ashley Madison stats I'm not using as in the aftermath of the 'hack' of the Ashley Madison site, it was discovered there were numerous fake accounts.) Regardless, I'm limiting inclusion of statistics in this post, reliable or not.
Inside ClubTOW, a member had once asked about success stories with the members of our board. Depending on how "success" is defined, the answers were variable (for those wanting out of their affair, success was achieving an end to the affair; other members have married their affair partner and define that as success). One member had the following response:
I know of at least 5 couples who were affair partners and are now married - one of them for over 20 years. All of them had D-Days and either left spouses or were told to leave. I can't say what percentage this is as obviously I don't know how many people in my social circle are/were having affairs in the first place.
As illustrated here, affairs typically aren't obvious in normal social circles; outside of a D-day occurring a divorce and subsequent marriage (to an affair partner) may fly completely under the radar.
(I've also known an affair couple who have now been married to each other for close to 30 years now; I don't recall many details although there likely was a D-day to realize they were affair partners to begin with.) I believe one of the statistics for successful marriages that start as affairs is (or was) 2.5%, or maybe it was 10%.
Is it accurate? I don't believe the statistic really matters as it doesn't take into account your own situation.
(That said, statistics are generally unfavorable to affair partners marrying one another.) 
Actual affair statistics are skeevy IMO: infidelity/affair statistics have got to be the least reliable statistics in existence due to the very nature of the subject and the social taboos against it. If polled, I believe many of us currently in affairs (especially if married) wouldn't fess up to it in the real world ... and if not fessing up to it, likely would matter not how it started, etc.
What do you all think about affair/infidelity statistics? Are they accurate? Do they even matter? *For those of you outside the U.S., Abraham Lincoln had been President of the United States (elected in 1860, and assassinated in 1865) ... dying well before the advent of the internet. Quotes attributed to him about the internet are satirical. 
