Quite often when an Atheist tells a Theist that they have a lack of belief in gods, it tends to lead to the Theist thinking they believe there are no gods. When our position is explained more, their knowledge of it switches to the opposite end, that all gods are possible, so why don't we believe in their god(s) then? Well, it would appear we have a new question on our hands.
"If Atheists believe gods are possible, why don't they believe in any known gods?"
To put it bluntly, we see no reason to. When we look at all of the gods currently "revealed" to man, we can't see a single one that, to us, makes any sense. That's not to say they don't make any sense at all, nor am I saying they are all false gods. All that means is that we see nothing that would suggest they are real while we see things that suggest to us that they are fake.
I'll put it this way. When we see gods depicted in holy books, we usually see ourselves in those gods. When we see decisions made by those gods, we ask ourselves if we would make the same choice given the situation described. Unfortunately, most of the time, we respond with a resounding no.
I can't speak for every Atheist, but I know what I would do if I was a god.
With ultimate power, being able to do anything possible and know everything, I would have two choices ahead of me if I wanted to create man. Do I make myself known to man or not? And what is the final goal of man? Is man meant to be a partner? A legacy? A friend?
If I were to make myself known to man, I would influence them significantly. There would be serious repercussions that I would have to actively counter or compliment. This would require intervening whenever man would go astray. Countering after the fact would be inefficient, so prevention would be the best method. Teach man what is wrong and why. If man continued to do wrong, further teaching would obviously be needed. Knowing everything, though, I would also know exactly how to teach man to never do wrong again without harming anyone.
On the other hand, if I were to not reveal myself to man, I must also be aware of the repercussions of this decision, something I would have no further control over. As such, I would have to make the universe and man in such a way as to bring them, eventually and efficiently, to their goal. Not revealing myself also means never intervening with man's progress. They would have to be by themselves, no one to gain knowledge from, no one to show them what is right and wrong. As I used my power to make a universe set on the right path, they would eventually learn everything they needed to know. However, before that point they would be stumbling through the dark. They would be trying to find any meaning to life they could, creating their own religions and faiths to explain their presence or relying on science to discover it for themselves.
In both cases, man would arrive at the goal set before them. Both would require full use of Omniscience and Omnipotence. Both require gods to exist. And, as I see it, these are the only possibilities for a benevolent god, one with good intentions for man or any other creation. For the first option to be real, I would have to have heard or seen god, regardless how I think. He would be an active, known force. My mind would be changed in an instant, before I harmed anyone with my stray, incorrect thoughts. I would still have the same amount of will I have now, I would simply be taught better.
Since the first option is not occurring, I must default myself to the other option. If any god exists at all, he or she must be taking a backseat to our development. This god would have set the universe in motion to be able to reach the desired conclusion as fast as possible, all on its own. This would mean that all of the gods we know about were all man made, not real gods.
Again, this is all my opinion. This is, however, my reasoning for believing gods can exist, but don't have to, and aren't any that man knows about. My opinion can change, if given a reason to, but that reason has yet to surface. Perhaps it will one day, but until that day comes I will continue to promote Atheism in a positive light.