In my world you can't be committed to an intimate relationship and not be monogamous. If the same is true in your world, you need to ask your partner if they want to be part of your world. Having a partner even consider anything else and ask me would make me re-evaluate the relationship; this is not something I feel like I would have to talk a soul mate out of. This is not a line in the sand for me; it is chiseled in stone. This is how I feel and I infer from your discomfort that you feel that way also. My apologies if I misunderstand.