Homosexuality is more than a sin, it's an abomination (God takes it a bit more serious than "gossip").
Up until the early 70's, homosexuality was slowed down by enforcement of sodomy and public decency laws (the good ole days when you could actually go into a public restroom to rid yourself of bodily waste and not have some pervert peeking at you through a glory hole). Since we no longer are living in the good ole days, God had to improvise with AIDS.