Why do many christians view as something sinful?

I just want to know why is it many christians view as something sinful , i mean i know the bible says we shouldn't be lustful and shouldn't have it outside marriage , but i mean why is it many christians take it as something that shouldn't be talked about even some saying that it only should be used to make children and using it for any other purpose (within marriage) is wrong but i mean if is such a bad thing then why did God make it pleasurable and why didn't he make some other way of reproducing.