. Incest laws are there to protect the gene pool from genetic abberation caused by the increased likelihood of harmful recessive genes producing unwanted results, and typically have prevented opposite sex relationships within close blood relatives. So having sex with a clone is most likely to involve a same sex relationship, which (in the UK at least) isn't illegal. It is pretty horrid mind you. I do seem to remember seeing a Jerry Springer with lesbian identical twin women that got it on with each other. *shudders*
Arguably, masturbation is "Excitation of one's own or another's genital organs, usually to orgasm, by manual contact or means other than sexual intercourse." If you're having sex with a clone, then that's sexual intercourse.
God damn it, I need to get out more.