Biblical Monogamy is a sham?

Hey! Peace all.

Im not a christian, but i love reading about all of it, and i come across the idea of monogamy a lot in the christian doctrine, especially around debates on homosexual marriages because it mentions how the bible says "Marriage is in between one man and one woman" instead of "a man and a woman" which gives the idea of monogamy.

But, what about Jacob/Israel marrying Leah and Rachel?

Do not claim that the bible had not yet been revealed since Moses is yet to come.
I believe in Jacob as a prophet of God, and Prophets have communion with God either directly or through mediation like angels and Gabriel. God wouldve let Jacob know that marriage is only between one woman and one man. How is it that Jacob still had two wives? and 2 more concubines? I thought we couldnt have concubines in modern day.

Second question is, also relative to homosexuality (You can only quote the gospels from the NT) why are laws like mixed fibres abolished since Jesus Christ came to fulfill them but the levitical laws of "sodomy" are not abolished? Why do you still not keep sabath or burn yourself at the stake but only invoke OT rules when it comes to homosexuality?

I do not intend to fight, i genuinely wanna know. Im sorry if my wording comes across as offensive, ive often been told but that is just how i talk, but let me tell you i dont intend any offense.