Many great men — kings, presidents, prime ministers and others — have had their women stray.
I know gay men who have platonic female partners, only because they have no interest in women.
People have always strayed sexually, it's the way of the world, the habit of mankind, the modus operandi of men and women who have wandering eyes.
Other sexual shenanigans occur when women marry men who are much older than them.
The motive is her wanting to hear more sweet words and be wanted again, and the opportunity presents itself when the other man presents his means.