Winter,
It seems odd the way you keep framing your questions. Whats odd about it is that what youre asking, by definition, is the answer.
Its like asking how would the color green be affected if yellow and blue got together? Well.. obviously, thats how you get green.
If society at large loses its ability to distinguish between what is moral and what is not, then the harm is in the same fact. By promoting the acceptance of promiscuous behavior, you change the dynamic of what was seen as immoral as now being acceptable and "normal".. thus "moral". This is harmful to any moral code as things that society once treasured as being sacred and good, now lose more and more of its meaning.
That is a bad thing. Challenging the merits of morality is always healthy, it always keeps our sense of right and wrong in check. However challenging the merits of what is moral and right through the examination of what others do wrong isnt really a defensible position. Thats the whole.."I stole because, everyone else is doing it".. thats not a defensible position to argue from.
For example.. right now we have laws and over-reaching sense of morality when it comes to adults sexually abusing children. Would you stand up and ask "what harm does it do to you if Jimbo molests his OWN daughter?"... "How is it any of your business if Frank molests his OWN daughter?".. "You know kids are having sex at younger ages now, theyre growing up faster.. how does it affect you if a man wants to have sex with a 12 yr old.. probably 10 of her friends are already having sex."
Do you see how the way these questions are framed and argued entirely misses the point?
kinda sounds a lot like "If my wife and I decide to have an open marriage, how can that possibly effect you and your wife? How is society harmed?"
Good to go?
SR