In the marital act, there are two functions: The generative and unitive.
Now, sex doesn't have to be only for its generative function. It is morally okay to have sex without the intention of having a child.
However, the generative function is still important! It plays a role in making intercourse a kind of act that couples can choose to express their love for each other.
Procreation is an objective good! We are told by God to "be fruitful and multiply."
In order for an act to be a loving act, it has to be geared toward an objective good.
Contraception on the other hand, is treating each other for mutual use, instead of treating the other as a person. The very nature of contraception is a direct attack on the generative function of intercourse. This turns intercourse into an act of use rather than an act of love.
When using contraception, you subordinate the generative function for the function of pleasure. The other person becomes merely a means for pleasure and not treated as an end in itself. Contraception denies the intrinsic value of the other person. To love someone is to say that is is good that they exist simply for the fact that they exist. Contraception, because of its treating the other as a means, cannot be love.
John Paul II said, "the innate language that expresses the total reciprocal self-giving of husband and wife is overlaid, through contraception, by an objectively contradictory language, namely, that of not giving oneself totally to the other."
It is no wonder that the use of contraception statistically leads to divorce!
John R.T. Lamont is his article On The Functions Of Sexual Activity states:
"Divorce was rare amongst both Catholics and Protestants when the members of these confessions rejected contraception as immoral, but became widespread after contraception was accepted and practiced."
The alternative to contraception is Natural Family Planning. NFP is open to the generative function of intercourse. Practitioners of NFP are willing to give up acts for pleasure in respect for the generative function of intercourse.
In NFP, you have loving affirmation of the other, rather than use that happens in contraception. It also opens up space in the marital relationship to express other forms of love. Practitioners of NFP gain a stronger awareness of their spouse which leads to greater communication between them.
For more info on NFP click here.
Here are also some Myths and Reality about NFP.