"It was the act of bodily union that achieved marriage... it was at all times lawful for any of the Eldar, both being unwed, to marry thus of free consent one to the other without ceremony or witness… in flight and exile and wandering, such marriages were often made." Tolkien then proceeds immediately to a discussion of begetting of children. So, "marrying thus of free consent" means that the elf-man and elf-woman involved agree to be life partners, and that there is no excuse for elf casual sex.