Yes, the use of artificial birth control, to avoid pregnancy, is a mortal sin. Always.
However the Church does not teach that you must have a ton of kids. What the Church says is that you can work within God's procreative design to plan your family.
Each couple should prayerfully consider if they can accept a child. Licit reasons to avoid pregnancy are serious financial, emotional, psychological, etc. If you decide that you cannot accept a child at a particular time, you can abstain from sex on fertile days.
Natural Family Planning (NFP) works with your fertility and remains faithful to God in practice. It is 99% effective when followed correctly.
http://news.bbc.co.uk/2/hi/health/6375261.stm
With NFP you get to know your body very well. You know the signs of fertility, and can abstain from sex at those times. This allows you to plan your family and the additional benefit is a healthier marriage, and closer relationship with God.
Sex is a holy act, a gift from God. It is both procreative and unitive, to remove either of these aspects is to sin. To block the procreative aspect and take just the pleasure says to God that you do not trust him, nor do you want him in your marriage or family. Birth control takes selfish approach to sex, which is contrary to the self-giving and life-giving divine purpose.
Here are some links:
https://www.youtube.com/watch?v=QtkXFMSPQ5o
https://ccli.org/
http://www.memoryhole.net/kyle/2005/08/natural_family_planning.html
http://www.catholic.com/tracts/birth-control