Well, if the couple is unequally matched in the sexual libido
arena, then yes... it can be one huge catalyst for affairs.
Imagine if everynight, you try your darnest to 'get some' but night
after night, you get rejected.
I gather for both men and women, being viewed as sexy and
attractive to their partners is a huge ego boost and it adds to
their overall feeling of well-being. (Don't forget, sex is ranked
as the #1 basic need in the Maslow's Hierarchy of Needs! 
)
If one is rejected day-in, day-out, and left going to bed squirming
uncomfortably, then, unhappiness sets in...
And then, when the next person comes along and makes them feel
attractive and desired... DANGER!!!
