As a guy who is old enough to have suffered through this, and have had female friends leave their s/o’s for other dudes, i can tell you that most of these situations occur because of a lack of honesty on the woman’s part.  On the one hand she likes having a “man” even though she is out there shopping/looking for a bigger better deal.  Someone said it best below, in the beginning everyone is interesting and exciting.  It is also hard for you to see this “new” person in all their glory/flaws when you  have the luxury of a relationship safety net.  Things you’d not necessarily accept in a r/s, you’d accept in all the euphoria of the exciting times with this new person and not see it for what it is.  From all I have seen over my 37+ yrs, most of the women that do this sort of thing end up in a far worse situation relationship wise because the ‘”new” men were not properly vetted.   Having the freedom to do your due diligence w/o hiding from your current partner is definitely important in making a well thought out decision.  because while dealing with someone new & having a spouse in the picture, the dissatisfaction u r feeling (whether real or imagined) with ur current spouse will be so intense that it will ultimately cloud ur judgment.  

Me personally if you want to date another man then just tell me. I’m a big boy, I can handle it. I rather you be straight up and honest as oppose to me walking in on you with your legs kicked up high in the air saying some nonsense like “It’s not what you think it is”.