A rational observer will notice that human beings are not naturally a pair-bonding species. In our culture, about 50% of married women and about 70% of married men will have extra-marital sex at some time during their marriage. Some sociologists and other researchers theorize the percentages would be much higher, if not for the religious connotation of "sin," the religion-based laws about adultery, and the stigma associated with peoples' natural sexual preferences. Monogamy over an extended period of time appears to be an unrealistic expectation, one that causes huge amounts of human suffering, pain, and of course, legal fees. For many married people, monogamy is a form of sexual slavery.
The institution of marriage is inherently flawed by the idealistic fantasy of monogamy until "death do us part." Why not improve marriage to become a secular contract of partnership that does not require monogamy? Instead, monogamy could be an option that couples could add to their contract at any time they want, especially during times when they want to have children.
An important part of a non-monogamous marriage contract would be each partner's responsibility to keep the other informed of his/her sexual contacts, with an emphasis on safe sex. An important part of freedom from monogamy is a reduction over time of the notion that marriage implies "ownership" of the spouse. One beneficial end result might be a significant reduction in domestic violence.