Simple! It is easy to rail at gays, a tiny minority whose "sin" is something most would never be tempted to commit. 

If they wanted a Bible based morality based on Jesus' teachings, they'd rant about gluttony, greed, lusting after others [porn], divorce, adultery.

But this would hit too close to home for those in the pews; these are sins most of them are tempted to commit and do!