[alnorth]

The reason for Monogamy is jealousy. That is not a negative comment, its the simple truth.

Even if you love someone but also have a complete lack of jealousy, you can make all the "hey I love you, but I don't mind, if you are happy then I'm happy, and you should feel the same way" arguments you want, but if your partner doesn't feel the same deep inside, you won't convince or argue the jealousy out of them. If you want an open relationship, you are limited to the very few people in the world (which doesn't include me) that have a similar lack of jealousy.

The reason for monogamy is simple, and not nearly as interesting or complex as the reason for marriage. "why do we have marriage" is a separate question. (ie you can love and marry someone but also both not be monogamous)