Catena Aurea on Matthew 5:27–28: Gregory the Great

But whoso casts his eyes about without caution will often be taken with the pleasure of sin, and ensnared by desires begins to wish for what he would not. Great is the strength of the flesh to draw us downwards, and the charm of beauty once admitted to the heart through the eye, is hardly banished by endeavour. We must therefore take heed at the first, we ought not to look upon what it is unlawful to desire. For that the heart may be kept pure in thought, the eyes, as being on the watch to hurry us to sin, should be averted from wanton looks.