In pure devotional service there is no question of sense gratification. Some people mistake the loving affairs between Kṛṣṇa and the gopīs (cowherd girls) for activities of ordinary sense gratification, but these affairs are not lustful because there is no material contamination. As Rūpa Gosvāmī states in his Bhakti-rasāmṛta-sindhu (1.2.285),
premaiva gopa-rāmāṇāṁ kāma ity agamat prathām
ity uddhavādayo 'py etaṁ vāñchanti bhagavat-priyāḥ
"Although the dealings of the gopīs with Kṛṣṇa are wrongly celebrated by many as lust, great sages and saintly persons like Uddhava hanker for such loving affairs with Kṛṣṇa." Śrīla Kṛṣṇadāsa Kavirāja, the author of Caitanya-caritāmṛta, has therefore said,
kāma, prema,-doṅhākāra vibhinna lakṣaṇa
lauha āra hema yaiche svarūpe vilakṣaṇa
"As there is a difference between iron and gold, so there is a difference between material lust and Kṛṣṇa's loving affairs with the gopīs" (CC.1:4:164). Although such loving affairs may sometimes resemble material lust, the difference is as follows:
ātmendriya-prīti-vāñchā-tāre bali 'kāma'
kṛṣṇendriya-prīti-icchā dhare 'prema' nāma
"The desire to satisfy one's own senses is called lust, while the desire to satisfy the senses of Kṛṣṇa is called prema, love of God" (CC.1:4:165).
The impersonalists cannot understand the principle of satisfying Kṛṣṇa's senses because they reject the Personality of Godhead. Thus they think God has no senses and therefore no sense satisfaction. But the devotees simply want to satisfy the senses of the Supreme Lord, and so they take part in the pure activities of love of Godhead. There is no question of lust in that category of pure transcendental love.
Lust leads to fruitive activity for sense gratification. There are different kinds of duties for the human being, such as political obligations, performance of Vedic rituals, obligations for maintaining the body, and social formalities and conventions, but all such activities are directed toward satisfying one's own senses. The gopīs, however, simply wanted to satisfy Kṛṣṇa's senses, and thus they completely gave up the conventional path of social restriction, not caring for their relatives or the chastisement of their husbands. They gave up everything for the satisfaction of Kṛṣṇa, showing their strong attachment to Kṛṣṇa to be as spotless as washed white cloth.
It is said that when conjugal affection between a lover and beloved comes to the point of being destroyed and yet is not destroyed, such a relationship is pure love, or prema. In the material world it is not possible to find this kind of love, for it exists only between Kṛṣṇa and His intimate devotees, such as the gopīs. The sentiment between the gopīs and Kṛṣṇa was so strong that it could not be destroyed under any circumstances. Kṛṣṇa praises the gopīs' pure love in the Śrīmad-Bhāgavatam (10.32.22):
na pāraye 'haṁ niravadya-saṁyujāṁ
sva-sādhu-kṛtyaṁ vibudhāyuṣāpi vaḥ
yā mābhajan durjaya-geha-śṛṅkhalāḥ
saṁvṛścya tad vaḥ pratiyātu sādhunā
"My dear gopīs, I am not able to repay My debt for your spotless service, even within a lifetime of Brahmā. Your connection with Me is beyond reproach. You have worshiped Me, cutting off all domestic ties, which are difficult to break. Therefore please let your own glorious deeds be your compensation."
(Narada Bhakti Sutra).

Bhakti means to serve Hṛṣīkeśa by the hṛṣīka. Hṛṣīka means senses. Kṛṣṇa is the master of the senses, and therefore, whatever senses I have got, the master is Kṛṣṇa, proprietor is Kṛṣṇa. So when our senses are engaged in the satisfaction of the master of the senses, that is called bhakti. This is the definition of bhakti, devotional service. And when the senses are engaged for sense gratification, not for the master, that is called kāma. Kāma and prema. Prema means to love Kṛṣṇa and do everything for satisfaction of Kṛṣṇa. That is prema, love. And kāma means everything done for the satisfaction of my senses. This is the difference. The sense is the medium. Either you do it, satisfy your senses, or you satisfy Kṛṣṇa's senses. But when you satisfy Kṛṣṇa's senses, you become perfect, and when you satisfy your senses, you become imperfect, illusioned. Because you cannot satisfy your senses. That is not possible without Kṛṣṇa. Hṛṣīkeṇa hṛṣīkeśa-sevanaṁ bhaktir ucyate (CC.2:19:170).
(Lecture on Bhagavad-Gita------2:9-------by Srila Prabhupada).

The gopīs are going to Kṛṣṇa, being captivated by the beauty of Kṛṣṇa, just like a young girl becomes captivated by seeing a very nice boy, or a nice boy is captivated to see the beauty of a girl. These are sense gratification. There is no prema; that is kāma. But the gopīs, they are going to Kṛṣṇa, superficially the same thing, like the young girls are going to a young boy, but they are going for Kṛṣṇa's satisfaction, not for their own satisfaction. That is the sublime. Therefore gopīs are so held in estimation even by Caitanya Mahāprabhu. Ramya kecid upāsanā vraja-vadhu-vargabhir ya kalpita: "There is no better type of upāsanā, worship, than it was conceived by the gopīs." Caitanya Mahāprabhu admitted that the topmost method of worshiping Kṛṣṇa is the type of worship offered by the gopīs. Topmost. Gopī-bhāva-rasāmṛtābdhi-laharī-kallola-magnau muhur vande rūpa-sanātanau raghu-yugau śrī-jīva-gopālakau. The Gosvāmīs were always thinking of the gopīs' service to Kṛṣṇa. Gopī-bhāva-rasāmṛtābdhi.
(Lecture on Bhagavad-Gita------16:4-------by Srila Prabhupada).