There are two types of forums (forums?) Open and closed. A closed forum is a bit like a church, where everyone agrees, everyone believes the same things. So that people can go and be surrounded by those who believe exactly like them, they can strengthen and congratulate each other for their beliefs. An open forum is more like a tavern, where everyone has the chance to talk and where you go to listen to a range of opinions and opinions.
Yahoo Answers is an open forum. No matter who can say anything, within the limits of decency (and these days not even). Atheists have exactly the same right to express themselves and defend their points of view as you. If you do not like it, you will find many forums on the Christians Only program on the Internet.