That statement comes from two of the most respected early Catholic theologians who were later made Saints.
Says St Augustine,
"Nothing is so powerful in drawing the spirit of man downwards as the caress of a woman and that physical intercourse which is part of marriage."
(Soliloquies)
"In Augustine's judgement – and subsequently that of the Church – sexual desire and gratification ("lust") had to be controlled, limited and confined. Libido was stigmatised as a sin, detracting us from God. In contrast, celibacy, chastity and virginity were lauded as being far closer to the perfection of god and were to be the choices of preference. Since such precepts severely threatened the continuance of the human race, passionless, matrimonial intercourse solely for the procreation of children remained permissible, though even this was a 'venial' sin. Premarital and extramarital sex clearly were sins, as was sex during pregnancy or after childbearing age. Even the harmless release of masturbation became a grave sin, the crime of 'Onanism'."
"Thomas Aquinas ... thought masturbation worse than rape (because 'unnatural', in the sense of unable to lead to conception). Indeed on this ground he considers rape, incest and adultery lesser vices than masturbation, homosexuality, anal and oral intercourse, and coitus interruptus. Again, he thought deviation from the 'missionary' position in sexual intercourse a serious sin, believing as he did that this made conception more difficult!"
(Daphne Hampson, After Christianity, p189)