It seems Zev was looking for a response from me with his post below, "Kinsey on Haredim".
My first response: Zev, you are big fat idiot, and nobody loves you.
Second, as far as halacha is concerned, polygamy is mutar. There are rules about polygamy (this isn't, after all, vietnam) but Yemenites were practicing this even into this century. There is nothing in the kesubah that we use today which prohibits this, as far as I know, and only cherem keeps ashkenazi men from marrying multiple women. I haven't looked into the matter, to be honest, but we could all perhaps pull a ginsbu and marry ourserves a harem of docile Persian teenagers.
More to the point, and to answer your question: No, it is not immoral. At least it wasn't for the Avos, and the way your question was framed, it can only be interpreted as attempting to refer to some universal extra-halachic morality (because as pointed out above, the halachic answer is obvious) - implying that if it is immoral today, it was immoral for them, and I think you're frummer than to try and imply that.
I could say more, but that'll do for now. Here's some unnecessarily transparent song lyrics:
Sonic Youth
"Diamond Sea"
Time takes its crazy toll
and how does your mirror grow
you better watch yourself when you jump into it
'cause the mirror's gonna steal your soul
I wonder how it came to be my friend
that someone just like you has come again
you'll never, never know how close you came
until you fall in love with the diamond rain
throw all his trash away
look out he's here to stay
your mirror's gonna crack when he breaks into it
and you'll never never be the same
look into his eyes and you can see
why all the little kids are dressed in dreams
I wonder how he's gonna make it back
when he sees that you just know it's make-belief
blood crystalized as sand
and now I hope you'll understand
you reflected into his looking glass soul
and now the mirror is your only friend
look into his eyes and you will see
that men are not alone on the diamond sea
sail into the heart of the lonely storm
and tell her that you'll love her eternally
time takes its crazy toll
mirror fallin' off the wall
you better look out for the looking glass girl
'cause she's gonna take you for a fall
look into his eyes and you shall see
why everything is quiet and nothing's free
I wonder how he's gonna make her smile
when love is running wild on the diamond sea
Have a good week everybody, and send some love.