I was confused when I first encountered the word “partner” in the context of referring to a spouse or the person one is in a romantic relationship with. I thought: “What partner? Business partner? Tennis partner? Oh, sex partner – that’s where the reference to partner comes from.”
A similar speculative etymological derivation can be applied to the US slang phrase “hook up”, which means to start a romantic relationship. The phrase arose to describe a situation in which a “hook” goes up something, specifically someone’s bottom.