Is non-vaginal intercourse sinful?

The Church teaches that the marital act must be completed inside the woman because this preserves the procreative aspect - it preserves the openness to life.

Genital activity outside of normal intercourse is not the marital act. That’s the point. The marital act must serve two aspects: unitive and procreative. The union initiated by the vows at the altar is physically expressed in the normal sexual intercourse of the marriage bed. Attempting to complete sexual intercourse outside of its context can be mortally sinful even if conception cannot take place due to surgery or pregnancy or old age.

However, foreplay (e.g., oral stimulation) leading to intercourse is allowed. Christopher West, in his book, Good News About Sex and Marriage (pg. 90-92), says “The acts by which spouses lovingly prepare each other for genital intercourse (foreplay) are honorable and good. But stimulation of each other’s genitals to the point of climax apart from an act of normal intercourse is nothing other than mutual masturbation. There’s no gift of self, no marital communion taking place at all. Nor are such acts open to conception.”