Hey there.
I know've a few people who're into the same things you are, and I'm good friends with people who identify themselves as furries. So I understand where you're coming from.
I do think Shezbut is right...suppressing both your fetishes is a quick route to resentment. Rather, I agree with what's been said in finding why this bothers her so much...I'd understand so much better if this were a matter of you looking at porn or something, but these are inanimate objects.

It's exactly analogous to if she had a certain toy she liked or something like that...I don't understand the problem, honestly.
That being said, understanding what her grievance is with this can go a very long way. I know you mentioned that you pleasure yourself to them...do you think it's possible that she thinks that you think the plushies are more attractive than her? She needs to understand, if that is indeed what she thinks, that it couldn't be further from the truth. That's like you being you being jealous that she's using a toy that happens to be larger than you. Surely she could see the analogy? And you mentioned her dressed up as a plushie could be a turn on...do you think you two could work something out (honestly, you could incorporate both of your fetishes, which might help ease her into it)? The key, I think, is for her to understand that A) Her and the plushies aren't mutually exclusive B) That this isn't a fetish at all unlike BDSM and C) That you love HER.
I wish you both the best, and I hope you can work something out.
My best,
Harley