How does the US justify its relationship with Saudi Arabia when half its population are in essence property i.e. slaves (women). Besides the oil, and if that is all how do we in the US reconcile our condemnation of Boka Haron, Taliban and so forth?
Saudi Arabia is one of the few stable governments in the middle east. Ultimately, stability is desirable- it allows those involved to produce more, consume less foreign aid and, of course, more stability means they can secure the area and avoid allowing extremist groups like IS and al-qaeda to grow. While it's true that many of their policies are seen as oppressive, this has to be taken in context with the surrounding area. Israel is another instance of a country which is supported by the US despite questionable actions- again, they exist in a volatile area and serve to stabilise it. The growth of stable, non-hostile states is a good thing in any unstable area, but especially in the middle east.
It's also worth mentioning that there is an ultra-conservative element in SA, Wahhabiism, which is kept from doing any real damage. The country was essentially founded that way- a deal between the Saudis and the Wahhabis. It is worth helping the SA government simply so they can keep a check on them and avoid them causing trouble, or even better, use their fanaticism to our advantage.
Another note is that Saudi Arabia are good trading partners. Besides the fact that stability and military ability help with this, if they keep their deals and don't default on debts, that encourages you to keep dealing with them. So, deal-to-deal, it just makes sense.
Morality doesn't come into it that much. It never does. While we often go up against states that abuse human rights, they are also often belligerent. Morality might help some justify government action, but in reality it's never going to be the primary motivator behind anything, especially not business.
Btw, it's important to note that the biggest arms deal in history was between the UK and Saudi Arabia, so it's hardly just the US.