Beyond the Dawn/YMMV

"But these ceremonies were not rites necessary to marriage; they were only a gracious mode by which the love of the parents was manifested,(8) and the union was recognized which would join not only the betrothed but their two houses together. It was the act of bodily union that achieved marriage, and after which the indissoluble bond was complete. In happy days and times of peace it was held ungracious and contemptuous of kin to forgo the ceremonies, but it was at all times lawful for any of the Eldar, both being unwed, to marry thus of free consent one to another without ceremony or witness (save blessings exchanged and the naming of the Name); and the union so joined was alike indissoluble. In days of old, in times of trouble, in flight and exile and wandering, such marriages were often made. (Morgoth's Ring)"
 * Eternal Sexual Freedom - Beren and Luthien engage in joyful sex before "approved" marriage; this offended many "orthodox" Tolkien fans who see such action as going against Tolkien Elven mores. Given that Tolkien certainly did not contemplate that option when his guardian kept him away from Edith, this probably did increase his in-grave RPM count a wee bit.
 * Beren and Luthien were married: