It's true that desires aren't justified by reason in and of themselves, but I don't think that's ultimately relevant (morally or otherwise). It's an empirical fact that we must act on our desires; when I examine my actions closely I notice that all of them can be traced back to some desire or combination of desires. From that empirically derived fact I posit that we can say the question "can reason alone justify the pursuit of our desires?" is a non-sensical question, like asking "is gravity justified in making objects fall to earth?"

I think a valid question along this line of inquiry is "are the actions I am taking in pursuit of my desires justified?". Actions are embodiments of ideas about how to act, and like other ideas can be deemed justified or unjustified through rational analysis. If an action embodies an idea that contradicts a known truth, it can be deemed unjustified.