Or more irrational.
This is an argument I've heard a-lot lately, especially from people calling themselves agnostic.
The reasoning is that we atheists believe that something doesn't exist though we have no evidence that it doesn't exist.
If I told you of a lollipop pooping pink rhino that I believe may exist though I have no evidence to present to you, or no evidence that you found credible or scientifically sound, would you be irrational for not believing in this rhino?
If 2,000 years from now your ancestors also did not believe in this rhino would they be irrational?
Do you believe in the Flying Spaghetti Monster? If not, does this make you irrational?
I can think of lots of things that you don't believe in and I can make up lots of things for you not to believe in.
Lack of evidence is a perfectly rational reason to believe that something does not exist.
An atheist may believe that there could possibly be other life out there in this big universe, and that this other life could be more advanced than we are currently. Atheists can leave room for lots of possibility in the undiscovered world, but while we work on discovery it is not irrational to let go of the old explanations that were made up in the absence of knowledge.
It was once believed that god "put the baby there." Now we know about sperm and egg and the humping. It was once believed that god responded to the sacrifice of animals. Now we know about the protein in their blood serving as fertilizer, we also know that poop works better. Would it be rational for me to continue believing that god is behind the other mysteries we haven't solved or is it more rational, as a scientific minded person to accept that there are unknowns?
To simplify and summarize let me ask you this; How the hell do you prove something DOESN'T exist? Why, that's just silly. You prove something DOES exist, or you prove that it's likely it DOES exist while you continue to look for absolute proof of it's existence.