The romance between Lord Krishna and Radha is known to everyone. She was the romantic lover of Krishna. Radha was the childhood friend and lover of Krishna.
Once, Sri Krishna got very ill. Krishna asked that he would be alright if, he would get the Charanaamrit of a true devotee. All the Gopis were asked but none of them offered. They were afraid of committing a serious offense, by offering the water with which they had washed their feet to Sri Krishna.
When Radha came to know about the situation, she said: "Take as much as you like. What does it matter if I have to go to hell as long as my Lord is released of His pain and ailment"! Radha offered the Charanaamrit with true heart. It is due to this fact; it is believed that Radha could not marry Lord Krishna. Radha loved Krishna from the core of her heart but still she offered Charanaamrit to Krishna to save him from his sickness.
Radha and Krishna were celestial beings and their love was eternal. Whether they were married or not, their love made them united for ever.