Normally, I would consider massaging & tickling as just playful bonding amongst friends... anything involving oral or genital contact is sex, period.
That said, I'd be full of it. My best friend from grad school is from South America & has that Latin character. She is affectionate (always a hug here, a peck on the cheek there) that often irked the prudes among us. She would often ask me to massage her feet when we studied together, and I would often oblige (hiding any sign I was aroused as best I could). [sadly, she isn't that ticklish]
This had been going on for a couple years, when I just started going out with someone. And when she saw me touching my friend's feet (by that point, foot massages became 2nd nature between us), she didn't like it one bit!
It was our 1st real fight. She had no idea (and still doesn't) that feet & tickling arouse me so, BUT that kinda contact was out-of-bounds for her. And quite frankly, she's right. I apologized & swore my allegiance to her feet. That episode ended. We broke up a while after that... she did end up being kind of a prude, especially in the bedroom.
Soo, to answer your OP, if it arouses you & it ain't with your woman ... it's cheating, whether she knows it or not.