Enforced? By whom? It's an agreement between two consenting individuals hence there is no room for coercion here.
Breach of 'contract' under such circumstances becomes an issue simply because it represents a serious breach of trust.
Traditional marriage itself is the institution that enforces monogamy. Our entire culture, religions, govts, media, social norms enforce it. To marry 1 person and only 1 person "at the exclusion of all others" is the underlying conditional statement. Countless stories, songs, myths, etc. glorify monogamy as "natural" and "universal".
The trust is there that when you marry another person, you will both be monogamous, because the institution of marriage is monogamous - and that is why you sign a marriage license or contract. The trust is broken when one or both parties can not uphold their side of the contract.
What happens is after a certain amount of time, one or both partners get bored and want to try someone/something new. Given that they are now in contract for life to this person, they end up either cheating, or repressing their desires, culminating in resentments, and other issues.
Most people today practice "serial monogamy" i.e. date someone new > break up > date someone new > break up > date someone new...
or
Marry for life > Divorce > Date > Marry for life > Divorce > Date > Marry for life > Divorce...