Possibly the best, most elegant summary of morality is the Golden Rule: do unto others as you would have others do unto you. It’s short, easy to understand, and easy to apply. And likewise, we can summarize immorality handily as the opposite of the Golden Rule: immorality is when you do harm to others who have done no harm to you.
Homophobia and discrimination against gays is exactly that. Homosexuality just means you fall in love differently than heterosexuals do. You have done no harm to heteros, but heteros seek to do harm to you. In fact, you’ve done no harm to any god, either. Those who seek to do you harm, when you have done none to them, are doing the exact opposite of the Golden Rule.
This is how we know that prejudice against gays is immoral, and is a discredit to any person, mortal or divine, who promotes it.