"reasons" for marriage have shifted too.
I don't condemn George Washington for owning slaves. Do I think it was right? No. But I have the advantage of having grown up in a time where I have a very different perspective than he had.
I'm not saying Brigham Young having been sealed to younger women is the same as slavery, but justifying/judging peoples actions today based on the actions of someone who lived 200 years ago is simply not reasonable.