Actually this is a bit out of context. I don't think it means any sexual partner. Remember Paul was talking to peoples with various relegions and beliefs, many area's may not have had a formal marrage, and they would have to be married after the fact once converted.
I wasn't finished with my morning caffeine fix yet and thought about that after I wrote it. What I was thinking was that some folks are going to use the word "partner" without considering the context; and nothing will matter to those folks as they'll read whatever they want into it.