Monistat is not a preventative, but I have used it while riding (I get YIs when I use antibiotics).

I'm assuming that you're sure it's a yeast infection, but I will add this: In replacing my saddle once, I didn't get the new one completely level. I experienced horrible irritation that I first mistook for a YI. You say this is a new bike. Is there any chance that it might be irritating you in such a way that it feels like a YI?