There is no specific law against it that I'm aware of. However in the late 70's or early80's there were several people who went to jail for shooting sex with bondage because it was considered obscene by the courts. Whether it is still considered obscene is open to question, but you'll probablye have to go to court to find out
This is in the US, btw. In europe and canada different rules apply.
If I were going to shoot it today, I'd talk to a good first admendment lawyer before I ever picked a camera and tied a knot.