No there is NO such thing as "no strings" sex (or relationships),because inevitably one of the two persons will feel more than the other,and as such "strings" will form whatever anyone might actually say.
For a couple of years, I had a no strings relationship with a good friend of mine. The rules and boundaries were understood and we both got something out of it - company, fun, nights out, a sympathetic ear, and of course, uncomplicated sex. That part of our relationship is over now (by mutual agreement), but the friendship endures and is strong as ever before.