Why is contraception a sin?

When God said be fruitful and multiply in Genesis 1:28, it was because he wanted to create a human race that he could love and cherish for eternity, creatures made in his image who can appreciate him, worship him, and share a love relationship with him forever. To have that, you need creatures, and to be special they have to be made in his image. God has free will and can choose to create; nothing holds him back except sin, which he could never do. So to say to God, no, I will not follow through with your plan, I am going to contracept and prevent those creatures made in your image from being made, is to attempt to thwart God's creation plan, and that is sin. Choosing not to marry is one thing, and then you should dedicate your life to the church some way; but if you do marry, you are telling God you have made a vow to bear children if it is possible for you, and it usually is. If you then break that vow through contraception, you have sinned against God.