I think that the thing that you are missing here, especially in this arguement that “if you are avoiding, why not use a condom ‘just to be sure’ that you do not conceive” is completely missing the point of what the marital embrace is each and everytime.
The marital embrace is the giving of ourselves to our spouse totally, fully, faithfully, and fruitfully. We are giving them everything that we have (just as Christ does for the Church, his bride) to our spouse. We hold nothing back.
Having said that, we must keep in mind that every act (every act) of sexual intercourse with our spouse must be:
#1 Unitive - uniting the spouses in the marital embrace giving of themselves to one another as described above.
#2 Procreative - always open to new life
From the Catechism:
2363 The spouses’ union achieves the twofold end of marriage: the good of the spouses themselves and the transmission of life. These two meanings or values of marriage cannot be separated without altering the couple’s spiritual life and compromising the goods of marriage and the future of the family.
Now getting back to what you are saying about avoiding a pregnancy, that is possible during the infertile times of the month. Those that practice NFP know when they are fertile and infertile. It is not a sin to have sex during an infertile time, you can do that, you are still open to the possibility of human life even if you are avoiding.
Again from the Catechism
2368 A particular aspect of this responsibility concerns the regulation of procreation. For just reasons, spouses may wish to space the births of their children. It is their duty to make certain that their desire is not motivated by selfishness but is in conformity with the generosity appropriate to responsible parenthood. Moreover, they should conform their behavior to the objective criteria of morality:
When it is a question of harmonizing married love with the responsible transmission of life, the morality of the behavior does not depend on sincere intention and evaluation of motives alone; but it must be determined by objective criteria, criteria drawn from the nature of the person and his acts criteria that respect the total meaning of mutual self-giving and human procreation in the context of true love; this is possible only if the virtue of married chastity is practiced with sincerity of heart.156
2369 "By safeguarding both these essential aspects, the unitive and the procreative, the conjugal act preserves in its fullness the sense of true mutual love and its orientation toward man’s exalted vocation to parenthood."157
2370 Periodic continence, that is, the methods of birth regulation based on self-observation and the use of infertile periods, is in conformity with the objective criteria of morality.158 These methods respect the bodies of the spouses, encourage tenderness between them, and favor the education of an authentic freedom. In contrast, “every action which, whether in anticipation of the conjugal act, or in its accomplishment, or in the development of its natural consequences, proposes, whether as an end or as a means, to render procreation impossible” is intrinsically evil
By using a condom you have taken out one of the essential purposes of the marital embrace, the procreative aspect. God is in control of our lives. Plain and simple. By using artificial contraception, you are telling God that He can be incharge of everything else in your life, but your sexual life, you will be in charge of. You will be the god of your sexual life. How wrong is that? God knows all about sex. God created sex!! He is the one that is in charge.