If Radha and Krishna aren't married, Why do we worship Them together
Once Radha asked Krishna why he loved Radha but married others. Lord Krishna gave a sweet reply of Radha's question. Lord Krishna said that marriage is only possible between two persons but not between one soul. We (Krishna and Radha) are one soul so our love will be eternal and divine but earthly marriage is not possible.
But Krishna made a promise to Radha - whenever anyone in this world will remember Me, they will always take your name before Mine, and never any of My wives, as a reminder of our unconditional and immeasurable love.