Should atheists be homophobes?

Typically they aren't. But consider this argument:

Premise 1: If God does not exist, then something like Blind Watchmaker Neo-Darwinian Evolution (hereafter just “evolution”) is a fact.

Premise 2: If evolution is a fact, then, objectively, my only purpose in life is to survive, reproduce, and spread my genes to the maximal extent (this premise is taken, essentially, from the mouth of Richard Dawkins).

Premise 3: Homosexuality makes it impossible to reproduce and pass on our genes. 

Premise 4: Therefore homosexuality prevents us from achieving my only purpose in life. 

Premise 5: Therefore homosexuality is wrong, and should be discouraged.