Have you announced your engagement to you family?
That's what society does in monoshipping. You bring your dating partner around to meet the fam. After a while of general dating, you announce you are engaged in contemplating a serious step and keep bringing them around as your intended fiancee person. This gives you and them time to try this person on in family gatherings more seriously than "just a guest" and more like "potential relative" -- get time to develop their OWN relationship with this important person in your life so they can become important to them to.
This has not been done in your case.
If you are coming out of the closet with a handfasting -- some people are not going to take kindly to it because you gave them no time or consideration even if they WANT to be welcoming to the new relative!
1) Because it is foreign and they have no map for how to behave even if they wanted to be nice and welcoming. You shoosh it on them without giving time to learn. They feel you are not treating them nicely. And you are not.
2) Because you did not prepare them or allow them time to make relationship with this stranger -- who now has to be what? The turkey carver at Thanksgiving? It's like eloping and then expecting all to be joyous about it -- they did not get to share in your joys along the way and now you want instant joyness? Most families aren't that flexible.
3) Because they do not accept you polyshipping because they do not like polyshipping. They cannot reconcile you (someone they like) doing something they do not like, so they throw the baby out with the bath water. That's hard to take. But we deal.
- If they break up with you? You tell them you are sorry, you value their relationship and if they ever want to try again your door is open. But no, you will not change. You love you you love. And it is unfortunate they want to end the relationship over who you choose to love.
- If rude to you? You can break up with THEM. There's nothing you can do there other than be firm about "You do not have to like polyshipping. But I will not have you talk smack about me or my loved ones. It does not flatter you. You can call me to apologize for your rude. We are not in right relationship until you do. Good day."
We teach others how we want to be treated.
Examine how you ARE treating your family. Before you rush off to worry about how they MIGHT treat you. Have you given THEM opportunity to rise to the occasion or not?
Can you be perceived by them as doing lies of omission? In hiding this person in your life? (You could have valid reasons like living in a place where hate crime could happen! But you still have to make nice with the relatives for not being out to them right away. Hopefully they understand and rally to your side and welcome the new relative. That would be the best scenario.)
If your polypeeps agree to being "Out" now -- each one call their own family and announce the engagement. You may or may not change the handfasting date or just hold another one later down that family IS invited to.
(I've been married to DH three times so far over the years -- handfasted, city halled, ministered -- different times, different people invited to bear witness.)
You write your own life story. You have the right to live as you wish.