[Avodah] [Areivim] Polygamy

[Moshe Y. Gluck]

On Areivim, we had the following exchange, regarding Tzaros:
R'n CL:
> > That to my mind is no casual naming.  The assumption embodied in the
> > name is that where you have more than one wife, you have trouble and
> > strife.  You
> > don't really need to say any more than that to indicate disapproval.
R' ZS: 
> Not necessarily; it may be a headache and an unhealthy situation if
> not handled correctly, but that's not to say it's inherently
> undesirable;
> there may be compensating benefits.

It seems to me that Chazal generalized when it came to human nature
intersecting with Halachah. For example, IIRC, it's a Mishnah/Gemara in
Eiruvin that says that if a man is in his daughter-in-law's house for
Shabbos (and he is also in the Techum of his own house) that he isn't Koneh
Shevisah by his DIL's house because DIL's and FIL's don't get along, and
he's keeping his options open - he might need to go home on Shabbos, in case
of a fight. Now, what if we have a fellow who gets along fine with his DIL?
Doesn't matter, the Halachah doesn't change. Same as with R'n CL's example -
we still won't believe the second wife to be Mattir the first wife even if
they are the best of friends. The Halachah doesn't change.
How do we interpret this? Are Chazal saying that the underlying human nature
is absolute? It might appear that the FIL and DIL get along, but there is
some predisposition to argument that will always remain? Or do we say that
even though in this situation the two are best friends, Lo Plug? 
Seems to me that R'n CL would pick the former, and R' ZS would pick the
latter.

KT,
MYG