Things I've heard as reasons:
They're selfish
They don't have self-control
It's unnatural to be monogamous Read this link to find out more about that
They want out of the relationship
There is something specific their regular partner doesn't give them
They're selfish
They don't have self-control
It's unnatural to be monogamous Read this link to find out more about that
They want out of the relationship
There is something specific their regular partner doesn't give them