The thing isn't whether or not it's considered sexual harassment; the fact is that it would be from most of us.
Here's the thing; this is very likely a sexual act to most of us, right? We're deriving a sort of pleasure from it that isn't necessarily natural, thus it is. It's more taking advantage of the fact that others do not consider this sexual, more than anything else. A lack of consent, whether or not their aware of it, would mark this as sexual harassment.
Now, here's the thing; this can also be used as a social means. IE, a conversation starter, you know? It really depends on the situation.
At any rate, I don't condone it in the manner that you're (OP) referring to. It isn't right. Probably, though, if the person was someone I was interested in speaking to, and I was in that sort of situation, then yes, that would be how I began it. Quick finger up the sole, read the reaction, follow up accordingly, etc.
But, at any rate, yeah, this is an invasion of anothers privacy just because this is the sort of thing I think most of us are into. I guess... well, what if someone walked up to you and rubbed your knee or elbow or something, in a similar fashion? You know?
Etc~
