I've always thought of masturbation of meaning one who has sex with one's self and have always equated mutual masturbation as sexual activity with another person, even if there is no penetration.
I in NO WAY think I'm correct on this matter, but just thought I'd throw that out there.
Thanks!
Gypsy - Betty Buckley
http://www.youtube.com/watch?v=tUN5XoB5vFs&feature=youtu.be