Another reason aside from the (blatent) its a sin aspect is we believe as Catholics that when we marry we are joining ourselves through Christ’s love as One flesh, husband and wife. We act out this marital vow of being One through the marital embrace. When we marry, we say to each other I love you totally and completely, and I want to give all of myself to you.
Just as Christ on the cross gave all of Himself for our sinfulness, we act out our Christ-like sacrifice when we give all of ourselves to our spouse through our marital embrace.
We believe that in using artificial birth control, no matter what kind, we are saying to our spouse that we want to give all of ourselves BUT our fertility to our spouse, and that we are SPECIFICALLY and DELIBERATELY blocking God out of our reltionship and out of one of the most important signs of marriage. Sex.
To put it in more everyday language, its like saying to your spouse “I love you so much, that I want to show you how much I love you. You can have all of me as a gift to you of my love. You can have whatever you want. But Tonight…you CAN’T have my sperm (egg, etc). I want to withold this from you.”

sorry but that was cute
Sex is our way in marriage to be completely united in One to our spouse and to Christ at the same time, by participating in the marriage sacrament. By using ABC we are witholding our fertility and thus part of ourselves from our spouse inhibiting this sacrament to be fully carried out the way Christ intended.
That is why we as Catholics not only believe it is a sin for US to use ABC, but as stated in Humanae Vitae, it is an actin that is “intrinsically” evil. This means that no matter what the cirumstances, when used for THAT purpose (that of blocking God’s creative act out of marriage through an artificial means), it is always a sin.
Again let me reiterate what others have said that using the pill for medical purposes is a completely different topic and should be addressed in another topic.
Please idoubtgod, I am so sorry for your loss thank you at least you are polite, but am very encouraged and feel very blessed that you have come here for answers, that is a great start. Not being Catholic you may never have heard of “The Theology of the Body” by Pope John Paul II. Is you’ve never heard of this please look it up. PM me and I will send you the book “Theology of the Body” for beginners by Christopher West for FREE. Just let me know if you want it. It is a very easy read and it may change your life, I know it has mine.
Please pray for me, and know that we are all praying for you, especially in this tough time of your life.
God Bless