Yeah, the explanation is that “sex” means “sex” and it doesn’t mean “sexual orientation” or “transgender status”. This seems pretty obvious. Why did 6 vote for seems more questionable, the Supreme Court is a judicial branch, not a legislative one.
The question that I think is still untested is what happens when you consider someone asexual or bisexual, as that isn't as simple a case to handle as just flipping the genders. If you fire a man for loving two people and fire a woman for loving those same two people, it doesn't appear you are discriminating based on sex. But given that such a relationship would also be considered poly-amorous bisexual and not monogamous bisexual, and given that the likelihood of having a business that only fired people for poly-amorous bisexual relationships and not for monogamous homosexual relationships is extremely small, I'm guessing it will be some time before it is tested in courts.
That's basically the argument. Discriminating against a woman who is in a relationship with a woman is discrimination against (certain) women, because if she were a male, in a relationship with that same woman, she would be treated differently.
"Well, that depends."
"What does it depend on?"
"..."