The Bible describes what were probably monogamous marriages. The Bible also describes holy men — prophets and patriarchs — engaging in polygamous marriages without condemnation.
The Bible mentions slavery. The Bible does not, to my knowledge, describe holy men being slavemasters.
The Bible never says that Abraham having Hagar for a (second tier) wife was a sin. That is pure interpretation. The Bible never says that Israel having (at least) four wives was a sin. That is gloss placed on the Bible by others, whom you apparently agree with, as is your right.
I just love it when Mormons defend polygamy. Thanks for being so honest and forthright in your beliefs!