First, let’s look at the verses your friend cites:
[Jesus speaking]: “But from the beginning of creation, ‘God made them male and female.’ ‘For this reason a man shall leave his father and mother and be joined to his wife, and the two shall become one flesh.’ So they are no longer two but one flesh. What therefore God has joined together, let not man put asunder” (Mark 10:6-9).
As the church is subject to Christ, so let wives also be subject in everything to their husbands (Eph. 5:24).
Neither of these verses say that husbands and wives sin by voluntarily agreeing to abstain from marital relations. The first citations speaks of the indissolubility of marriage and the second speaks of a wife’s relationship with her husband. (In the second citation, your friend does not consider the possibility that a husband and wife could mutually agree to refrain from relations for a specific reason.)
Paul, in his letter to the Corinthians, states that a man and woman have mutual authority over each other’s bodies (cf. 1 Cor. 7:3-4). He also says that there may, at times, be pressing reasons why a man and woman might refrain from relations (cf. 1 Cor. 7:5). He was envisioning a temporary circumstance but, as he was speaking to a general audience about the ordinary conditions for a husband and wife, he obviously did not have in mind the unique relationship of God’s own chosen parents (cf. Luke 1:43, 2:48). But Paul’s recognition that circumstances exist in which it is not sinful to mutually agree to forego marital relations shows that foregoing relations is not, in itself, inherently sinful.
Brethren of the Lord
Celibacy and the Priesthood
The Perpetual Virginity of Mary** by Br. Anthony Opisso, M.D.
Refuting the Attack on Mary by Fr. Mateo