I'm mad at my boyfriend for adopting a big dog when I already have a cat that is terrified of large animals
You don't think the cat will ever get used to the dog?
I see the dog as a threat to my cat
That's definitely not something you do without discussing it with your partner first. I think the dog has to go.
I can't believe he did that!
Yes, getting the big dog was an important decision. I can see why you are upset. This requires a very serious discussion between the two of you.