Actually, if someone really believed that, it would not be offensive to compare that belief to belief in God. The key difference is that when you compare belief in God to belief in Pixies, you are comparing their belief to something that no one actually believes.
(To be fair, I've met people who believed in pixies. Weird folks. Very weird.)
(inspired, begins thinking out loud)
When you compare belief in God to belief in pixies, what you are saying to the believer is that his beliefs, which are shared by the majority of people with whom he interacts on a day to day basis, at church, home, work and elsewhere, are identical to the beliefs held by those very strange people who talk to themselves on streetcorners.
From a strictly rational view of the beliefs themselves, you are right. However, the simple matter is that most people who believe in God are intelligent, rational, good citizens, while most people who believe in Pixies are strange folks who can't hold down a job and never really seem to grow up. That's what makes the comparisons offensive.