Monogamy Rocks.
Practiced monogamy would virtually eliminate STD's.
Practiced monogamy, as a general principle, produces the most stable offspring who become the most productive members of society.
Practiced monogamy teaches us what real love is. If you believe that love is nothing more than a mask for sexual, animal instinct, then this point will make no sense to you. When you give your sexuality entirely and unreservedly to one and only one other person, you take a step of humility and risk. You are yielding control over a part of your life that typically controls the lower animals and many humans. Without that yielding and commitment, most people will struggle to understand real love, which in its highest form has nothing to do with sexuality.
On a practical level, people tend to define freedom as "the ability to indulge my desires." That is not freedom. I begin to experience true freedom for the first time when, in the face of desires that are in and of themselves natural, I can, as an act of will, choose to say, "No." Then, for the first time, I am not driven by my own desires. I am driving them. For most people, there is no better way to achieve this freedom without challenging the strongest non-essential animal desire of all (sex) head on and choosing as an act of volition to redirect it into a higher love, a love of life-long, focused commitment.
By saying "No" to all others, I say "Yes" to my wife in a way that is powerful and unique. That is love.