If you sense that someone is either very philosophically deep or has some moral issues about sex, then its not a good idea to describe Rādhā as “Krishna’s girlfriend.” In those cases it is better to describe her as the female manifestation of divinity, the goddess epitomizing divine beauty and love. Describe her as the “personified bliss of Krishna,” and explain that their relationship with one another allows divinity to experience and enhance the fullest extent of divine bliss, which would otherwise only be latent within it.
If some question arises whether Rādhā and Krishna are married, you can explain it like this: They are two aspects of one entity – the potent and potency – so they are “married” in the deepest, truest sense of the word. But they express their love for each other as through they are not married because that makes their experience much more thrilling, exciting, and dangerous, more precious and rare, and more selfless and primal.
If some question arises over what sort of “example” this sets for humanity, just point out that setting an example for humanity is the purpose of many other avatāra, but not the purpose of Vrajendra Nandana Shyamasundara Krishna. The purpose of his avatar is to attract us to the raw, wild, supreme beauty of the most primal and original ānanda.
If the person still finds Rādhā and Krishna to be immoral, point out again that Rādhā and Krishna are one being, more married to one another than any husband and wife could possibly hope to be. But they are *playing* out their love *as if* they were not. Adulterous people in our realm cannot very honestly make the same claim (that one is the potent and the other is the potency, and they are they are thus eternally two halves of the supreme whole). Therefore adulterous people cannot use Krishna’s behavior as license for their own. However, even adulterous people benefit by hearing about Rādhā and Krishna, for they *can* see that Krishna and Rādhā “do it better than anyone else possibly can,” and so even their adulterous or sexually adventurous spirit can attract them to want to know more about Rādhā and Krishna’s love and perhaps even develop a strong desire to play some role in it.
In all cases, everyone should be introduced to Śrīmatī Rādhārānī Devī – but with the proper sambandha, the proper conception of who and what she truly is.