My (23m) girlfriend (21f) wants me to use a massive toy on her during sex but I feel uncomfortable

I guess this is half relationship advice and half “am I the asshole”.

My girlfriend and I have been together for about 6 months and having sex pretty regularly for the duration. Now (seemingly to me) randomly she brings out a massive toy (we’re talking like massive, like inhuman levels) and asks me to use it on her during sex. Now I don’t want her to feel bad about what she’s into, but I’m not using that thing on her. It makes me physically uncomfortable, I lost the mood the moment I saw it.

She doesn’t seem to understand that to me this feels like she’s calling me inadequate, and even if I know that’s not what she meant I still feel that way. I’m not trying to control what she does by herself, but when I’m involved I don’t want that toy to be.

To be clear, this isn’t the first time we’ve brought toys into the bedroom. Pretty regularly I use a small vibrator on her and we’ve experimented with other (more regularly sized) vibrating dildos.

Is there any way I can get her to understand what I’m feeling? Am I the asshole for not doing it?