My limited understanding of epidemeology (see, I can't even spell it :-) )
is that people aren't really living any longer than they ever did, they just
aren't dying as young. In other words, when the life expectancy was 40
years, that didn't mean that 40 year olds were really old people expected to
die soon, it meant that more babies died during childbirth or from childhood
illnesses thereby lowering the AVERAGE life expectency, not the actual age
of death from "natural causes". It has only been in the past decade or two
that medical advances have actually been allowing a significant number of
people to be old at a greater age (was that even close to being clear?) and
its really only a matter of a few years change- from something like 73 to
something like 77.
This is far too recent a development to have effected things like patterns
of monogamy. I also doubt that few people have decided they won't marry at
age 25 because they are willing to spend 48 years with somebody, but
certainly not 51 years.
Now I will fall back under the influence of my lurking meme.