Question
If Radha and Krishna aren’t married, why do we worship Them together, like Sita-Rama?
Answer
Besides the Brahma Vaivarta Purana mentioned in the other answer, the 39th Chapter of the 9th Book of Devi Bhagavatam also mentions the origin of both Radha and Lakshmi from Krishna:
4-33. Nârâyana said :-- Of old, in the beginning of the Prâkritik Creation, from the left side of Krisna, the Supreme Spirit, appeared in the Râsamandalam (the Figure Dance) a Devî. She looked exceedingly handsome, of a dark blue colour, of spacious hips, of thin waist, and with high breast, looking twelve years old, of steady youth, of a colour of white Champaka flower and very lovely. The beauty of Her face throws under shade millions and millions of autumnal full moons. Before Her wide expanded eyes, the midday lotus of the autumnal season becomes highly ashamed.
By the Will of God, this Devî suddenly divided Herself into two parts. The two looked equal in every respect; whether in beauty, qualities, age, loveliness, colour, body, spirit, dress, ornaments, smile, glance, love, or humanity, they were perfectly equal. Now she who appeared from the right side is named Râdhâ and she who came from the left side is named Mahâ Laksmî. Râdhâ wanted first the two armed S’rî Krisna, Who was Higher than the highest; then Mahâ Laksmî wanted Him. Râdhâ came out of the right side and wanted first Krisna; so Krisna, too, divided himself at once into two parts. From His right side came out the two-armed and from his left side came out the four-armed. The two-armed person first made over to Mahâ Laksmî the four armed One; then the two armed Person Himself took Râdhâ. Laksmî looks on the whole universe with a cooling eye; hence She is named Laksmî and as She is great, She is called Mahâ Laksmî. And for that reason the Lord of Râdhâ is two-armed and the Lord of Laksmî is four-armed. Râdhâ is pure Aprâ kritic S’uddha Sattva (of the nature of pure Sattva Guna, the illuminating attribute) and surrounded by the Gopas and Gopîs. The four-armed Purusa, on the other hand, took Laksmî (Padmâ) to Vaikuntha. The two-armed person is Krisna; and the four-armed is Nârâyana. They are equal in all respects.
Therefore, Radha & Lakshmi are on the same level and hence she is worshiped along with Krishna just as Lakshmi is worshiped with Lord Vishnu & her incarnation Sita is worshiped with Shri Rama.
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