Homosexuality isn’t intrinsically an atheist issue. Some atheists support gay people and some don’t.
Atheists tend to support the the gay and LGBTQ community more often than not because once theism is removed from the equation, there’s no good reason to oppose gay marriage within the context of atheism versus theism. Atheists are more 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. LGBQ individuals, as a minority, are victimized frequently alongside atheists.
Atheists do not have any doctrine to follow or holy book telling them that homosexuality is a sin or that they need to be put to death. Although some will deny it, religion is one of the main causes, if not the main cause for the persistence of homophobia in society today. In fact, I would say that for atheists who are homophobic, much of this stems from the culture they are apart of that has this influence due to religion. This does not mean that religious people are homophobic or against gay right, indeed many are not. However, it cannot be denied the role of religions (especially Islam and Christianity) in creating a homophobic society.
Another very important reason that the majority of atheists are also for LGBT rights is that the majority of atheists live in the Western world, where the majority of people are also for LGBT rights. Simply put, you won’t find too many atheists in Saudi Arabia because they will be killed. Atheism is also far more common in more educated populations and in more secular societies because the societies themselves are not theocratic and you will find that in these societies tolerance of LGBT communities will be much higher.
Of course, and this disturbs me, there are still atheists who are homophobic, but they don’t really have any legs to stand on to come to this position and quite simply, most atheists do not hold the belief of their religious counterparts that homosexuality should be a sin.
So yes, the majority of atheists are also for LGBT rights.