My two cents
I might be able to put some perspective on this, since I am gay and have tickled plenty of str8 guys.
Straightness and gayness are orientations. Whether you get turned on by tickling a guy's feet or a gal's, it has no bearing on your orientation. Tickling is what turns you on.
If you are starting to feel strange about how you feel when you tickle a guy's feet, it's because you've been trained, as we all have, that a guy who gives any sort of physical closeness to another guy is queer. That's why a few of the responses show the closedmindedness you would expect.
There's not one thing about what you did that shows any type of sexual encounter--closeness, yes, but sexual, no. If you, presumeably, are turned on by tickling someone, the tickling itself will probably get you off regardless of gender.
Your interest in tickling as an activity may be strong enough that you may have an orgasm in the presence of another guy, but it will not change your orientation. Sexual responses to stimuli are more fluid than orientation. A woman could tickle my nipples until I cum, but I will never be straight--ever. It's simply a matter of knowing that another man makes me feel whole and complete.
There are many, many str8 guys out there who are closeted about their enjoyment of tickling other guys, because of the concerns you expressed. I'm glad you posted about it, because it gives another chance to say to those guys out there that you are not alone and you are not weird. It also does not mean you are going to "become" gay.