Allowing marital contracts to stipulate "expiration dates" allows couples to form temporary unions, potentially circumventing the need to divorce. Or, for example, to perpetuate their union until their children reach maturity.
This should be, and in many cases is, up to the interests of those in the relationship. Everyone has their own boundaries and should form relationships that are mutually respectful of the boundaries of those involved.
Society is made up of human beings, many of whom are emotionally (or ideologically) attached to the status quo. If the monopoly that lifelong monogamy has had on marriage were threatened, there would inevitably be conflict. The degree of this conflict is unknown, but may outweigh the benefits.
Criticisms against Polygamous marriages exist. According to one study "Women in polygamous marriages showed significantly higher psychological distress, and higher levels of somatisation, phobia and other psychological problems. "
It would benefit even more with regulating the adoption laws that the LGBT lobbies are pushing for. Same sex marriage is not the same as heterosexual marriage and children are not objects one can just want and own.