Were the rules of morality altered?

Following is a short list of some of the pleasures the Torah permits but were proscribed over the last few thousand years thanks to the accumulation of Jewish piety.

1 - Chicken pizza (At least according to Rav Yosi Haglili, a first-century sage, who allowed fowl and milk to be eaten together.)

2 - Kitniyos on Pesach

3 - Multiple wives

4 - Concubines.

5 - Premarital sex (Hard to prove, I know, but I think there's an early consensus that this is permitted, technically, so long as the woman has been to the mikva)

6 - Slaves

And how's this for bizarre - if I, today, were to take a concubine, or a second wife, or a slave, my morality would be denounced, and leading the pack of denouncers would be the sort of people who swear and insist that morality never changes!.

It's really a puzzle for us relativists. The evidence of history is that we've always been content to let morals shift and slide, even - sorry, no especially- within the major religions. 

So how do the people who say that morality is absolute and never changing explain this?  Why can't I take a slave? Were the rules of morality altered?