If Radha and Krishna aren't married, why do we worship Them together, like Sita-Rama?
Srila Prabhupada is very careful that we understand the relationship between Radha and Krishna very carefully; it's not like some mundane love affair between a young boy and young girl. There are several ways to look at it, but first we must understand this point.
Srila Prabhupada points out in one lecture, that
". . .the love between Radha and Krishna is called parakiya. They're not married as husband and wife. Radharani appears to be the wife of someone else, but She and Krishna were friends from childhood. Radharani could never forget Krishna. She used to come to Krishna and stand before Him. And He was playing. They were Kisora-Kisori - boy and girl - but without inebriety.
"Here, the boy and girls mix, and there are so many abominable, distressful things, which increase their material bondage. But between Radha and Krishna the friendship between boy and girl is there without inebriety. Krishna had so many gopis (cowherd maidens) as His girlfriends, but there were no contraceptive pills. (laughter) That is the beauty. Here, the so-called love is lust. And there (in the spiritual world), that (lust) is the highest. Here, the same thing has become its perverted reflection. Just like in a reflection of a original tree on the water, the topmost part appears to be the lowest.
"Similarly, in the spiritual world, the highest, topmost level of love is parakiya. Parakiya means love - not by marriage, but by friendship. There, it is pure. There's no inebriety. "Perverted" means the topmost thing has become the lowest. Here (in the material world), parakiya, loving another's wife or husband, is most abominable; it is adultery."
On the other hand, there's another story: When Krishna was a small child, Radharani was asked to accompany Him home. In Their childhood forms, Radha and Krishna were walking together in Bandiravan. They stopped to rest under a banyan tree, and, by Their mystic potency, They manifested youthful forms. Lord Brahma was asked to be the priest for the wedding and guests arrived. Lord Brahma performed the marriage ceremony, with all the demigods in attendance. During the wedding ceremony, Radharani’s sari was tied to Krishna’s dhoti, as is the custom in Vedic weddings. When the ceremony was over and the guests had left, They assumed their childhood forms again. Radha gave Krishna to Mother Yasoda and then walked home.
In this way it is accepted that Radha and Krishna are actually married. However, in order to make their pastimes more relishable, Radha is "so-called" married to Abhimanyu. The intrigue of Them (Radha and Krishna) meeting in secret and planning and plotting against Jatila and Kutila always makes Their pastimes more relishable.
On still another level; Krishna is actually the only male (purusha - "enjoyer"), and everyone else is prakriti or female. So, Krishna is actually everyone's pati ("husband"), but especially He is every woman's husband. In fact, anyone who's married to anyone except Krishna is committing adultery!
I hope this is helpful.