Hare Krsna,
Generally, people go to the demigods for material benefit, as indicated in Bhagavad Gita 7.20:
kāmais tais tair hṛta-jñānāḥ prapadyante ’nya-devatāḥ
taṁ taṁ niyamam āsthāya prakṛtyā niyatāḥ svayā
“Those whose intelligence has been stolen by material desires surrender unto demigods and follow the particular rules and regulations of worship according to their own natures.”
In this pastime we see that the gopīs of Vṛndāvana were performing Kātyāyanī-vrata, a vow dedicated to Goddess Kātyāyanī (a form of Durgā) to attain Śrī Kṛṣṇa as their husband. Does it imply that they are (a) worshipping a demigoddess for (b) some material benediction? In the case of Vraja-gopis, both these factors are incorrecṭ.
We cannot say that the gopis worshipped Kayayani devi for some material benedictioṇ. Their intent was not material but purely spiritual—to serve Kṛṣṇa in the most intimate mood of madhurya-rasa (conjugal love). This is stated in Śrīmad-Bhāgavatam 10.22.4:
“Each of the young unmarried girls performed her worship while chanting the following mantra. “O goddess Kātyāyanī, O great potency of the Lord, O possessor of great mystic power and mighty controller of all, please make the son of Nanda Mahārāja my husband. I offer my obeisances unto you.”
“The Pracetās received benediction from Lord Śiva, and as a result they attained the shelter of the lotus feet of Lord Viṣṇu. This is real benediction. The gopīs also worshiped Lord Śiva in Vṛndāvana, and the lord is still staying there as Gopīśvara. The gopīs, however, prayed that Lord Śiva bless them by giving them Lord Kṛṣṇa as their husband. There is no harm in worshiping the demigods, provided that one’s aim is to return home, back to Godhead.” (4.30.38 Purport)
Therefore, it is not at all wrong on the part of the Vraja-gopis to worship Katayani devi. As long as the end goal is to obtain Krsna, such worship is valiḍ.
“Generally, people worship goddess Durgā for some material benediction. Here, the gopīs prayed to the goddess to become wives of Lord Kṛṣṇa. The purport is that if Kṛṣṇa is the center of activity, a devotee can adopt any means to achieve that goal. The gopīs could adopt any means to satisfy or serve Kṛṣṇa. That was the superexcellent characteristic of the gopīs.” (Krsna book, 22)
The second point regarding Katayani devi being a demigod is addressed elaborately by Srila Visvanatha Cakravarti Thakura.
“The goddess Durgā mentioned in this verse is not the illusory energy of Kṛṣṇa called Maha-maya but rather the internal potency (cit-sakti) of the Lord known as Yoga-māyā… We can thus understand that the internal and external potencies of the Supreme Lord are personified as Yoga-māyā and Mahā-māyā, respectively… To confuse the Yoga-maya Durga with the Maha-maya Durga is a mistake. Furthermore, although the pastimes of Vraja are on the platform of pure transcendence (suddha-sattva), they somewhat resemble material pastimes in appearance. Therefore, there is no fault in the gopis’ worship of Durga or Maha-maya. Independent thinkers who think that the name of Durga and Maha-maya refer exclusively to the external potency of the Lord have no chance of getting even a touch of the breeze carrying the scent of prema on their body. This is from Vaisnava-tosani by Śrīla Sanātana Gosvāmī.” (SVT commentary on SB 10.22.4)
Therefore the Vraja-gopis were not actually worshipping a demigod by Yoga-maya – the internal potency of the Lorḍ. This shows that their worship is completely spiritual and there is nothing material in their mode of worship and intentioṇ.
As an interesting sidenote, it is mentioned in Brhad Vaisnava Tosani (Srila Sanatana Goswamiś commentary on the 10th Canto) that according to Padma Purana, in a previous birth these girls had been sages during the time of Lord Rama. From that context, their desire to obtain Krsna as their husband is completely appropriate.
All glories to Srila Prabhupada!
Krsna Katha 