Sex is a wonderful thing, a gift of God for man.
It is a very good activity, and rightly pleasurable, since its ultimate end is one of the most noble and beautiful ones man can achieve during his life: the conception of a new person.

Man is not a mere animal. For him sex is a lot more than just a mechanical means for reproduction. This is undeniably the natural goal of sex, but it is not the only one: we speak also of the unitive purpose of sex, which is the deepening of the relationship between the married couple to a new level; a manifestation and a strengthening of the bonds of love which unite them both. In this unitive purpose, pleasure is also included; by giving and receiving pleasure from each other, husband and wife lead a happier and more fulfilling existence.

The great error of modernity with regards to sex is to separate its purposes.
On the one hand, Puritanism sought to eliminate the unitive purpose, bringing sex down to the level of a mechanical duty.
On the other, liberals seek to eliminate reproduction.

When a couple uses contraception, they are effectively frustrating the natural purpose of sexuality. And that is always a sin!

They fool themselves, believing that the unitive purpose may be fully realized without the procreative one; but that is false. Sex becomes an increasingly selfish activity, in which each partner seeks only their own pleasure, and the other one becomes a mere means for the fulfilment of their own lust. This is exactly what is happening in today's world.

Man has abandoned the procreative purpose, and the unitive purpose is being lost as a consequence.

Yes, I am against all forms of contraception, that is, the use of means which seek to frustrate the procreative end of any particular sexual relation.