I see three separate issues.
1) In my own experience, my gut is always right. The times I've tried to rationalize my way out of it and overridden my gut, it turns out I should have told my brain to STFU and followed my gut. I don't know your gut, but my guess is it's trying to tell you something.
2) BUT, that doesn't mean a veto is the best response. If your partner loves and trusts you, then you should be able to speak with him about your gut feeling and he shouldn't blow it off as jealousy. Maybe it's something about the way he's handling the situation. Maybe it's something about communication. In other words, it's not necessarily a "she's bad news, send her away" feeling... it could be a "something is wrong here, we need to fix it" feeling.
3) And since you already know something is wrong, i.e. you want to work on your partnership before adding people in the mix, and your partner is pushing to move things along faster than you're comfortable with, that's a good place to start.
So in other words, I wouldn't ignore the gut, but I wouldn't assume it's all her wrongness either. You need to move forwards from a place where you feel comfortable, and if he wants this all to work, he's got to respect that. If they're serious about each other, they'll both still be there after you and him work things out. If she can't wait for that, then she's not really serious about him. If he can't wait for that, then he's not really serious about you.
“As I am sure any cat owner will be able to tell you,
someone else putting you in a box is entirely different
from getting into a box yourself.” —bisexualbaker