Ok,

In order to try to find the biblical definition, I got an online strong's definition of the greek word that was translated as lust in Matthew:

epithumeō
ep-ee-thoo-meh'-o
From G1909 and G2372; to set the heart upon, that is, long for (rightfully or otherwise): - covet, desire, would fain, lust (after).

Notice, this does not mention the fact of it being a specific person. I did a search for lust in the entire bible, and most of the definitions were similar, something involving desire, longing, things like that. YOu can long for something non-specific, like, using your own example, you can long to suck some non-specific penis and according to this it would be considered lust. Some references make a distinction that the desires are strong, but most of them do not. In most biblical examples, this term was in references to two people, Since the bible did not address the specific cases we are talking about here, you have to try to in my view apply the above definition to this specific case. The bible says that lust as defined by what I just mentioned is sin. So, if you can masturbate while not desiring or coveting anything, then it is not lusting. But again, how many people actually do that?