my doctor once explained it this way--
in times gone by--most marriages didnt last very long. The husband would very likely die young--either worked to death, killed in battle or by disease. The widow was then free to find another marriage mate.
or...the wife might die in childbirth--leaving the widower to find another mate. etc.
Ive been married 4 times--and its nothing to do with a taste for wedding cake.