Darowyn, you suggest a meaninig that is the opposite of what I had imagined. You say that Lord Thomas asks Annie to remain his lover until his wife gives him a son. In this case, "that will end our love" means that the love that is going to finish is the love between Lord Thomas and Annie. I tought that the sense could be that Lord Thomas was asking Annie to wait, and be again his lover after the birth of his heir. In this other case, the love that is going to finish would be the love between Lord Thomas and his wife. I think your interpretation is more likely...