where do we get the idea that its sinful?
That is a teaching handed down from the very beginning.
Up until the 1930’s ,
all Christians, including all Protestant denominations, agreed with the Catholic Church’s teaching condemning contraception as sinful.
Contraception is wrong because it’s a deliberate violation of the design God built into the human race, often referred to as “natural law.” The natural law purpose of sex is procreation. The pleasure that sexual intercourse provides is an additional blessing from God, intended to offer the possibility of new life while strengthening the bond of intimacy, respect, and love between husband and wife. The loving environment this bond creates is the perfect setting for nurturing children.
Birth control has been around for millennia. Scrolls found in Egypt, dating to 1900 B.C. As to Christ, contraception was so far outside the biblical mindset and so obviously wrong that it did not need the frequent condemnations. Scripture condemns the practice when it mentions it. Once a moral principle has been established in the Bible, every possible application of it need not be mentioned.
The biblical teaching that birth control is wrong is found even more explicitly among the Church Fathers, who recognized the biblical and natural law principles underlying the condemnation.
Here is a good, in depth discussion of the subject:
catholic.com/tracts/birth-control
And here is an audio of a q/a.
catholic.com/radio/shows/…n-encore-27078