What makes someone agree to be the 'other woman'? And what's the fallout?

What do Angelina Jolie, Claire Danes, and LeAnn Rimes have in common? They've all been rumoured to be the "other woman", providing on-the-side action for seemingly happy, married, or attached men. Widely publicised, the outcomes of their alleged affairs were similar, with all three nabbing their taken men for themselves - at least for now. But while celebrity "other women" are often known as sexy seductresses, in reality, it can be a very messy affair.

So what motivates a girl to sleep with a guy who's already taken?