Because, prior to Roe v Wade, who'd of thunk it that abortion, arguably a type of murder, could be protected under, of all things, privacy laws.
Yes, and a lot of state Supreme Courts have been legalizing gay marriage on those exact grounds. However, on a federal level, the Supreme Court does not get involved with marriage laws, even if they are illegal discrimination. For example, the states could outlaw interracial marriages, and unless the state Supreme Courts ruled that as racial discrimination, the Federal Supreme Court would not get involved with it (wouldn't, not couldn't).
As for, specifically, what case it was, give me some time; I'll have to redo my research, but I'll get it to you eventually.