Let me add my welcome to the forum. I have trodden at least a part of the path you are starting out on right now, and I don't envy you - discussions like these can be very tough.
If your partner is interested in learning more, point them at this forum - there are also many other online resources where folks can educate themselves and find out just how "out there" they are.
Some people find solace in meeting other folks who are making the poly relationship style work for them - that can sometimes really help to allay the fears.
"Listen, or your tongue will make you deaf." - Native American Proverb