Homosexuality isn't intrinsically an atheist issue. Some support gay people, some don't. Other metrics exist as to whether a particular atheist does support homosexual issues, or not.

Atheists tend to support the LGBTQ community, more often than not, because, once theism is removed from the equation (and thus forming a secular framework), there's no good reason to oppose gay marriage, for example, within the context of atheism versus theism. Atheists are likely to adopt Secular Humanism.

Also, atheists tend to find themselves side-by-side with other minorities in defending their rights, so we tend to be allies against many religious groups. LGBTQ individuals, as a minority, are victimized frequently, alongside atheists.