I am sorry that this feels bad for you just now.
I do not think it is disloyalty really. If you had found a new partner, then he would have become grandpa 2 or whatever in the eyes of the GC. I know it feels like a pain, but that is probably because you are still hurting from the break-up, so this happening has a greater significance to you than it need.
I cannot imagine that anyone has let this deliberately happen to hurt you; but it feels hurtful to you. Don't let this weaken or muddy your relationships with your children - that would be a double whammy, and you can do without that.
Please try and enjoy making a good relationship with the lovely GC, regardless of what they call you. The love is no less, never mind the name. 
Another attempt by Trump to end the war with Iran


