"Sex, Hermione," Enola flushed. "Sex Magic is on a par with Life and Death Magic in terms of raw potency. The emotional releases involved, the very fundamental nature of the energies called upon ... it's one the most natural forms of magic there is. And, in old rites, a Bedding Ritual seals the Marriage Bond after a wedding ... and there are some traditions that also draw on the sexual energies from the nominated witnesses ... the Maid and Best Man ... to enhance that.  If Nev and I had sex at the same time as you and Harry, our energies would infuse with yours, making the Bonds between us all that much stronger and more intimate."