Gill - here is a distinction I would like for you to consider.
Do animals such as a giraffe or pelican have natural, or god-given rights? Let's remove god-given from those animals, since we are fairly sure they do not worship any gods. So we're left with natural rights. Do animals that are not human, have any natural rights at all? If so, what are they, and why, and where did they come from?
Here's my point, and possibly an explanation of where at least some atheists might come from. Atheists do not believe that humans are any more special than any other animal. That is usually a distinction reserved to believers, who think that their god created us, separate and distinct from the other animals. That is simply false.
And because humans have the highest level of human-defined intelligence, we decided to grant ourselves "rights", regardless of whether we describe them as god-given, or natural. We "think" we have rights, because we thought of them. And there may be a naturally good reason to have "rights", but there simply is no guarantee, what with serial killers, rapists, wars, genocides, and lots of other lovely forms of intrusion upon the "rights" of an individual.
But this is why we organize, and make governments, and laws. We need them. We would have a harder time staying alive, and surviving, if we just let anarchy reign. And notice too that over time, we can clearly see the pros and cons of different forms of government. We can also see the tyranny of forcing ideologies upon people without their consent. This happens in religious laws, as well as secular.
Anyway, do you see where I'm headed with this?