It's not absolute, women tend to be more manipulative than men to get what they want.
That's it.
Since as men, we don't have the advantage when it comes to manipulation and a lot of other factors in a relationship, we'd better be practical and always make sure that we use any advantage that we can have.
That includes keeping back ups.
The moment a woman knows that you're all she has, bad things are bound to happen.
It works both ways of course, I always need to make sure that I'm better than my competition.
That's how healthy relationships are built. I don't recommend obvious and childish moves like "I'm seeing an ex-f*** buddy tonight, you can trust me!" or flirting with other girls in front of her, that's just partner abuse.