I just find it interesting linguistically; "masturbate" primarily brings to mind sexually stimulating one's self (a reflexive verb, "I masturbate" means "I masturbate myself") -a solitary activity. But at the same time it can also have the meaning of sexually stimulating someone else, yet not denoting sex, because the stimulus is generally manual. I guess you could, however, masturbate someone with your feet, other body parts, or some tool. So we end up with this somewhat strange definition of masturbation as sexually stimulating one's self (primarily), or another person, but only so long as the stimulation of another person is done manually, with a tool, or some other body part that is not the anus, mouth, vagina or other orifice. Perhaps the meaning of masturbation is more about the pleasure of one party as opposed to the pleasure of all parties involved -regardless of the number involved. Or put differently, the "one way direction" of pleasure; thus one can masturbate 2 people at once by manually stimulating them, but other sexually activities would be defined as group sex. (Congratulations to anyone who actually read the entire post)