Marital intercourse for the sake of procreation has no fault attached to it, but for the satisfying of lust, even with one's husband or wife, for the faith of the bed, is venially sinful; but adultery or fornication is mortally sinful. Moreover, continence from all intercourse is even better than marital intercourse itself, even if it takes place for the sake of procreation. But even though continence is better, to pay the dues of marriage is no crime, but to demand it beyond the necessity of procreation is a venial sin, although fornication and adultery are mortally sinful.
It should be noted that Paul's viewpoint is somewhat different ...
Paul writes that marital intercourse should be consistent within the marriage, ... and that neither party should withhold from the other, except for mutually agreed upon periods of prayer and/or fasting, ... and even then encourages the couple to come back together in a timely manner so as to avoid the temptations of Satan.1 Corinthians 7:3 Let the husband render unto the wife due benevolence: and likewise also the wife unto the husband.
4 The wife hath not power of her own body, but the husband: and likewise also the husband hath not power of his own body, but the wife.
5 Defraud ye not one the other, except it be with consent for a time, that ye may give yourselves to fasting and prayer; and come together again, that Satan tempt you not for your incontinency.
-A8