Tirukkural-On Virtue-Not Coveting Another's Wife-Kural 148

Add as a preferred source on Google

The character of refraining from amorous glances at his neighbour's women,Is really the outstanding virtue of a righteous man.

Parithiar would explain that the bravery of a man who kills enemies on the battlefield, is not as great as that of man who has self control enough not to look amorously at his neighbour's wife. That really is the courage of manliness, because in that case he overcomes the inner enemy of his own lust – according to Parimelalagar. Virility is not associated with men of feeble self-control, who indulge in illegitimate desires.

This sentiment is brought out in other words in 'Jeevaka Chintamani'.

The following passage in 'Kambaramayanam' is also relevant:
Kalarir karandhurai kaamam aadhiyaam
Ullurai pagainyaruk kodhungi vaalvaeno

(Kambaramayanam-Mandhiram : 18)

This injunction to keep clear of the internal enemy called lust is in line with the teachings of Jesus, for it is recorded in the New Testament, 'Ye have heard it said, “thou shalt not commit adultery" But I say unto you, whosever looketh on a woman to lust after her hath committed adultery with her already in the heart'.

(matt 5: 27-28)