A MLCer is usually acting in a very erratic and uncharacteristic way. The affair [which may last even in MLC] is usually with someone that the person you know wouldn't look twice at.
A WAS more typically picks someone that, while they may be different from the spouse, has some points that the person you know would admire or like.
Many counter examples of course.
I would also say that not all WAS stay permanently away. The London Times did a survey a year or so ago, and found surprisingly high number of couples who got back together, or the WAS wanted to and the partner had moved on/wouldn't have them back. Not a statistical survey to place reliance on, but long marriages in which both people were happy a lot of the time are surprisngly durable.
I suppose what I am saying is just because one patrner leaves teh marriage doesn't mean that it is over. It may be. But not all affairs last, and sometimes the grass isn't greener, and they realise this . . . whether MLC or not. And sometimes it is.
