When marriage is analogized to the relationship between Christ and the church (or God and Israel) — God or Christ figured as the bridegroom and Israel or the church as the bride — it generally follows a monogamous model.
However, when the marriage is cast to reflect the relationship of the individual believer with God, it must perforce tend towards polygamy. Thus, in Jeremiah 31:32, God can lament that though he was "a husband to them," the people, individually and not just as a people have been unfaithful.
I was reflecting on this the other day in relation to the contrast between those of a more "catholic" ecclesiology versus those of a more "evangelical." (Pardon the imprecision of these terms.) The former sees salvation as taking place within the ark of the church, to such an extent that the old saying, "extra ecclesiam nulla salus" (there is no salvation outside the church) becomes a dominant theme. One is saved by becoming part of the corporate "bride" — and there is only one bride for the one Lord. The latter tends to see salvation as a more personal affair; in some cases rather radically so, the church perhaps taking the office of a kind of matchmaker, rather than the bride herself.
This might also relate to the general frangibility of protestantism.
Tobias Stanislas Haller BSG