除非 - "unless" / "only if"?
I recently encountered this sentence in a show: 除非佢肯同我成親。 (Standard Chinese, from the subtitles: 除非他肯跟我成親。）
I'm used to translating 除非 as "unless," but in the context of the show, I believe the sentence means "[I will do it] *only if* he is willing to marry me." Dictionaries tell me that 除非 can mean both “unless” and “only if.” I'm confused about this, because in English, these two meanings are opposites of each other. "Unless" basically means "only if NOT."
“...only if he’s willing to marry me.” = only if he WILL marry me.
“...unless he’s willing to marry me.” = only if he WILL NOT marry me.
My question is, if I see a sentence with 除非, how can I know whether it means "only if" or "unless"? Do I just need to guess the meaning from the context?