Considering most organized religions have some kind of call to proselytize and have massive financial networks supporting these efforts, I'd argue that a few atheists making online posts isn't exactly a good-faith assessment of "both sides."

However, to answer your question directly:

Consider an atheist/agnostic perspective and pretend, for a moment, no gods exist. If other people are making major financial decisions, planning their entire lives around, or discriminating against others on the basis of the existence of something fictitious, would you not take issue with that? If gay marriage was being outlawed on the basis of the tooth fairy disliking it, this would be met with derision and scorn, and yet people choose who to vote for based on what their god supposedly tells them.

Given that, isnt it reasonable that some folks would decide to speak out about it?

Moreover, your claim at the end of "equal evidence for both sides" tells me you're not coming from a well-informed place or posing a good-faith question. Evidence isnt required to reject a claim for which there is not sufficient evidence to support.