Wow, fascinating discussion. This is an odd place to make my third post, but I think I can contribute something here, as I haven't seen much of a defense of m/m tickling from someone who's actually interested in it, save for ttgore, whom I totally agreed with. So I'll offer an introspective into my own sexuality as example.
Ok, first off, I've been into tickling my entire life. I'm currently 31, and since I was 5 or 6, I've fantasized about tickling and being tickled, mostly the latter. Tickling always turns me on, no matter the context. So if a girl at a party gives me a quick tickle on my stomach, for me it's very exciting and it essentially feels as if she's grabbed my crotch. I try not to let it show, but this is how it feels for me.
I consider myself to be quite straight, if that makes sense. I find women attractive, and I thoroughly enjoy "vanilla" sexual activity with women. I don't find men attractive, and I honestly have no interest in kissing a guy or having sex with a guy. The thought does not turn me on.
Now, up to the age of 25 or so, I kept my tickling interest entirely focused on women. I say "kept", because in retrospect, this required a certain amount of active resistance on my part. I would stick to fantasies of f/f or m/f tickling, and even seeing f/m tickling weirded me out a little, because I'm generally focused on the what the 'lee is feeling/experiencing. I did, and still do, find this to be a significant turn-on. I would occasionally run into stories or pictures of guys tickling other guys, and I guess the idea always interested me, but I repressed it because it seemed gay.
Then, one day, I just kind of decided to let go and mentally experiment with the idea of being restrained and tickled by a guy. Turns out the thought was extremely stimulating to me. For a while, it was more exciting than "straight" tickling, mostly because it felt so new I suppose. Now, I'm actively interested in all forms of tickling, including m/m and f/m. I generally still prefer females, but I have chatted with men about tickling, I think about it often, and I am seriously considering trying to meet up with a guy to be tickled at some point. The thought of this turns me on immensely, and I'm sure I'd be turned on if I were being tickled by a man. However, I'm still not interested in taking it further with a man. I know it's possible I may still be repressing something, but at this point, I think I've honestly allowed myself to imagine thinks like that, and I'm pretty confident they're not for me.
So a pertinent question, given my situation, is whether I should be considered "gay" or at least bisexual because of this interest. I'm not sure what to say about this, because the labels are hard to apply precisely. Maybe I'm straight for sex but bi for tickling? I think a satisfying explanation is that I'm sexually attracted to women, but that I'm also sexually interested in the feeling of being tickled, whoever is doing it to me.