Da Vinci Folio A. 10 r.
Da Vinci Folio A. 10 r. Red bracket to the right side of the quoted text (which is written in mirrored form). (Source)

The art of procreation and the members employed therein are so repulsive, that if it were not for the beauty of the faces and the adornments of the actors and the pent-up impulse, nature would lose the human species.

[L’atto del coito e li membri a quello adoperati son di tanta bruttura che se non fussi le bellezze de’ volti e li ornamenti delli operanti e la frenata disposizione, la natura perderebbe la spezie umana.]

Leonardo da Vinci, artist
Leonardo da Vinci (1452-1519) Italian artist, engineer, scientist, polymath
Notebooks, De Anatomia, folio A. 10 r. [tr. McCurdy (1939)]
    (Source)

Windsor Anatomical manuscript A., folio 10 r / R. L. 19009R (Source (Italian)). Alternate translations:

The act of procreation and everything that has any relation to it is so disgusting that human beings would soon die out if it were not a traditional custom and if there were no pretty faces and sensuous dispositions.
[tr. Brill (1916), after Freud (1910)]

The act of procreation and anything that has any relation to it is so disgusting that human beings would soon die out if there were no pretty faces and sensuous dispositions.
[Variant tr. Brill (1916), after Freud (1910)]

The act of procreation and everything connected with it is so disgusting that mankind would soon die out if it were not an old-established custom and if there were not pretty faces and sensuous natures.
[tr. Tyson (1961), after Freud (1910)]

The act of coition and the members employed are so ugly that but for the beauty of the faces, the adornments of their partners and the frantic urge, Nature would lose the human race.
[tr. Dalwood (1962) after Bataille (1957)]

The act of copulation and the members employed are so repulsive, that if it were not for the beauty of faces and the adornments of the actors and unbridled passion, nature would lose the human race.
[tr. Armstrong (2013), after Nancy (2009)]