Krishna with Radha is the symbol of the Divine Love. The flute is the call of the Divine Love.
                                  

Radha is the personification of the absolute love for the Divine, total and integral in all parts of the being from the highest spiritual to the physical, bringing the absolute self-giving and total consecration of all the being and calling down into the body and the most material nature the supreme Ananda.
                                  

...the image of Krishna and Radha. ..has nothing to do with sex. The true symbol for it would not be the human sex-attraction, but the soul, the psychic, hearing the call of the Divine and flowering into the complete love and surrender that brings the supreme Ananda. That is what Radha and Krishna by their divine union bring about in the human consciousness. ..
                                

...under the stress of temperamental variation the poetry of the Vaishnavas puts on very different artistic forms in different provinces. There is first the use of the psychical symbol created by the Puranas, and this assumes its most complete and artistic shape in Bengal and becomes there a long continued tradition. The desire of the soul for God is there thrown into symbolic figure in the lyrical love cycle of Radha and Krishna, the Nature soul in man seeking for the Divine Soul through love, seized and mastered by his beauty , attracted by his magical flute, abandoning human cares and duties for this one overpowering passion and in the cadence of its phases passing through first desire to the bliss of union, the pangs of separation, the eternal longing and reunion, the lila of the love of the human spirit for God. ...This accomplished lyrical form springs at once to perfect birth from the genius of the first two poets who used the Bengali tongue, Bidyapati, a consummate artist of word and line, and the inspired singer Chandidas in whose name stand some of the sweetest and most poignant and exquisite love-lyrics in any tongue. The symbol here is sustained in its most external figure of human. passion and so consistently that it is now supposed by many to mean nothing else, but this is quite negatived by the use of the same figures by the devout poets of the religion of Chaitanya. All the spiritual experience that lay behind the symbol was embodied in that inspired prophet and incarnation of the ecstasy of divine love and its spiritual philosophy put into clear form in his teaching. ... Another type is created in the perfect lyrics of the Rajput queen Mirabai, in which the images of the Krishna symbol are more directly turned into a song of the love and pursuit of the divine Lover by the soul of the singer . In the Bengal poetry the expression preferred is the symbolic figure impersonal to the poet: here a personal note gives the peculiar intensity to the emotion. This is given a still more direct turn by a southern poetess in the image of herself as the bride of Krishna. The peculiar powet of this kind of Vaishnava religion and poetry is in the turning of all the human emotions Godward, the passion of love being preferred as the intensest and most absorbing of them all.