Those who don't believe in God can also "sin against themselves" as the bible describes someone who knows what is right but makes the wrong choice.
Sin is doing something you know is wrong, if you don't "know" homosexuality is a sin then you can't commit the sin. This is relativist of course, but that is how the bible talks about those who don't abide by biblical morality, and still describes the wrong behavior as "sin".
Unfortunately in the modern world sin is only associated with a religious violation, rather than a moral violation general to all humans.
edit: (well, maybe not unfortunate, as there are plenty of other words to demonstrate a wrong choice)
Wiki goes FAR beyond fear.
Homophobia is a range of negative attitudes and feelings towards homosexuality and people identified or perceived as being homosexual. Definitions refer variably to antipathy, contempt, prejudice, aversion, and irrational fear.
Its a classic liberal tactic to expand a definition of a negative word to apply it to anyone who doesn't agree with them and that's exactly what wiki is doing.
Our Constitution was made only for a moral and religious people. It is wholly inadequate to the government of any other.