This letter is saying that it's not a sin to get married right? Because Paul didn't think anyone should get married because the world was about to end, like any second.

I don't think it by any means over turned the convention of multiple wives and concubinage.

He also says there's no such thing as male and female in Christ right? So if two Christians want to get married in Christ it doesn't matter what their pre-marriage genders were, because they're one in Christ right?