Hi atanu,
I have a bit more isatheist an a basic knowledge of Hinduism, so I must agree as to its complexity.
And Buddhism, though it strives for something simple, has certainly acquired a vast and complex array of scriptures and schools throughout Asia and beyond to achieve that end.
But back to your main point. I agree that atheists understand what it means to believe. We can believe in friends, in loved ones, and in a better future. Some atheists believe in supernatural facets that are not deities (e.g. an atheist Buddhist might still believe in rebirth). So I would say that belief in a god or gods is not a requirement to understanding belief more generally. As someone who believed for a very short time in my childhood in the Catholic beliefs I was taught, and who was a pantheist in my early 20's after having a special experience, I happen to understand very well what it means to believe in a deity or deities. But I've met atheists who have never believed in a single deity their entire lives, so it varies. I do not see a problem with calling a lack of belief disbelief, though it is with the understanding that disbelief is a lack of belief rather than some kind of anti-belief.
Numerically, we could say that theism is a positive integer (e.g. 1 for monotheists, 2+ for dualists/polytheists, perhaps the set of all positive integers for pantheists). By comparison, the atheist would be represented as the number 0. In this analogy, it would not be applicable to have negative integers. So with this analogy, it's not opposites we are talking about with respect to atheism vs theism, but rather the presence (i.e. positive integer) or absence (i.e. zero) of belief.