Verse 1.40

Sanskrit text, Unicode transliteration, Word-for-word and English translation of verse 1.40:

अधर्माभिभवात् कृष्ण प्रदुष्यन्ति कुल-स्त्रियः ।
स्त्रीषु दुष्टासु वार्ष्णेय जायते वर्ण-सङ्करः ॥ ४० ॥

adharmābhibhavāt kṛṣṇa praduṣyanti kula-striyaḥ |
strīṣu duṣṭāsu vārṣṇeya jāyate varṇa-saṅkaraḥ || 40 ||

adharma–irreligion; abhibhavāt–due to having become predominant; kṛṣṇa–O Kṛṣṇa; praduṣyanti–are polluted; kula-striyaḥ–ladies of the family; strīṣu–when the womanhood; duṣṭāsu–become corrupted; vārṣṇeya–O descendent of Vṛṣṇi; jāyate–are born; varṇa-saṅkaraḥ–indiscriminate intermingling of the castes (brāhmaṇa, kṣatriya, vaiśya and śūdra) resulting in uncared for progeny.

O Kṛṣṇa, when a dynasty is overpowered by irreligion, the women of that dynasty become degraded. O descendant of Vṛṣṇi, when women become degraded and unchaste, the result is unwanted progeny.

Commentary: Sārārtha-Varṣiṇī Ṭīkā

(By Śrīla Viśvanātha Cakravartī Ṭhākura; the innermost intention of the commentary named ‘the shower of essential meanings’)

It is irreligion which causes them to engage in unchaste activities.