Medhātithi’s commentary (manubhāṣya):
It is the nature of women that they make men fall off from their fidelity: by associating with men, women would make them deviate from their vow.
For this reason the wise are never ‘unguarded’; i.e., they shun women from a distance;—‘unguardedness’ would consist in touching her and so forth.
It is in the very nature of things that a young woman, when touched, produces a disturbance in the mind; and this mental disturbance itself has been interdicted, to say nothing of other vulgarities.