Of the eight kinds of marriages defined in the Sastras (brahma, daiva, aarsha, prajapatya, gandharva, asura, rakshasa, pishacha), the marriage based on Kanyadaan (common to the first four types), was hailed as the ultimate of all "giving" or daan. It was considered the ultimate desire-free act that could possibly be performed by a householder (grihasta). The state of innocence and purity of the young girl who has no sexual desires (yet), who is the symbol of everything beautiful and tender is "given away" into the keeping of "Mahavishnu Himself" (the groom is given the duty of caring for the bride just as Vishnu cares for the world). The brahmavivaha was ordained primarily for the brahmanas and not the other castes. In this scenario, there is no desire at the socio-economic level to profit from. It was purely the union of two individuals who were united in the phyical realm to persue the spiritual path. The blessing was also given for spiritually-oriented children to be born of that union who go beyond the material plane.
The kshatriyas were encouraged to perform daiva (where the parents like the family of the groom/girl), aarsha, for political gains (gains for the larger society or community, not familial - to be noted), prajaapatya (for honor), gandharva vivaha (love marriage where partners choose each other - therefore no more "kanyadaan" in play).
Asura (for monetary purposes), rakshasa (resulting after rape/ by force: shotgun weddings of the wild west!), pishacha (under the influence) were highly frowned upon but permitted for certain vaisyas or merchants, under extreme circumstances. They were also encouraged to contract marriages like the kshatriyas with parental consent, and so was it for the shudras. This generic classification did not mean that the other castes could not perform a brahmavivaha. It is one thing to write in a law book and not expected to perform but another to say is not allowed to perform - which was not the case.
In the context of CB, while tradition would have demanded a brahma vivaaha for a Raj prohit, owing to Dev's love for Radhika, it became a Gandharva type but got the blessings of a Brahma vivaha. So, the current story line proves it- as far as rads and dev are concerned their "gandharva vivaha" has already taken place. That was the kind of marriage that Dushyanth had with Shakuntala.
I welcome readers to share more based on their knowledge of the scriptures.