I’d say it also depends on the purpose of the kiss. Was there any sexual/romantic element to it? Or was it like maybe just a dare, she has no particular aversion to kissing another girl and figures why not? That’s not really cheating. Now if you have this underlying current of curiousity and are really turned on by the idea (or the act), that’s another thing altogether.
But bottom line, really the only thing that matters is if he thinks its cheating. Rule of thumb if you’re dating, no matter what YOU think the status of your relationship might be, and no matter what YOU think constitutes cheating or not cheating, you have to get on the same page about it BEFORE you do anything that could be a gray area. You could think you’re just dating someone and he could think you’re in a relationship, and if you kiss someone else, then even if you think “I never said we were exclusive”, he’ll be hurt by it…you can be right and still do someone wrong.
For example, I was once out at a club with my wife and there was a drunken bachelorette party going on, this one girl with a dildo in one hand, wearing a suck for a buck T-shirt with life savers all over it asked if she could “grab my junk” because of a bet. I had no particular desire to have my “junk” grabbed by a drunken slutty stranger, so had I allowed the transaction to occur, I would have derived no particular stimulus from it, so I would have not felt like I was cheating if I’d done so, just like I was doing someone a favor.
But I felt, my wife really should have some input on this. I told the young woman to talk to my wife, my wife told her to grab someone else’s junk, end of story. I don’t know that my wife would have thought of it as “cheating” per se, but regardless of what label she’d have put on it, it wouldn’t have been kosher. Conversely however, I know her well enough to know that she has zero sexual interest in other women, but if she were to kiss another woman to say win a bar bet or something, I’d have no problem with it.
Bottom line, know your boundaries, know your partners boundaries, and if in doubt, don’t do it unless you ask first.