Yeah thats some pretty good points you bring up. The christiantiy has truths although in the 1500's ( or around then) people were very religous and feared their christian god, yet girls betweent he age of 9-14 were often married off to men over the age of 20. So perhaps it wasn't so much christianity as it was changing views on society that made sex "wrong".
I'd also stongly disagree with "we all have mroals so why do we need the goverment to regualte them for us". Quite simply we may all have morals but some are likely to be very very different so laws are created to punish peole who,for e.g, find murder morally acceptable.