How flexible is "the line?"

You all know what I'm talking about...sexual harrassment. Something happened in the office the other day that made me start to think...again. Where's the line?? Every work environment has its policies, but every work environment also has its own personality and its own norms. Something that many in one office may call harrassment most, or all, in another may not...right? So who draws the line. I would give an ultimately subjective answer here...whoever is on the recieving end gets to decide whether s/he feels "harrassed." But that still leaves a gray area the size of Montana...

So here's the situation...I work in an office that's extremely laid back. There are about 70 of us in the company, our boss brings his dog to work, we wear jeans and flipflops to the office on most days and shorts on Fridays. We party together, we drink together, we make fun of each other. We're generally playful (in an asexual way)...throwing things at each other, making paper airplanes, ripping on eachother in emails...you get the picture, not the most mature environment, we get our work done and have fun. However, last week a female coworker had crawled under her desk to plug something into her computer. Our manager happened to come out of his cubicle at the time and quickly reached back in, grabbed a handful of rubberbands, and handed some to me. He then proceeded to shoot them at her backside... I kind of watched in awe, thinking the line had DEFINITELY just been crossed. At this point, a male coworker turns from his desk w/ a response something like "What ARE you doing?" The manager laughs and says, "It's ok, I'd do the same if you were there." The female coworker's reaction was to laugh it off and shoot a few back, seemingly entirely comfortable with the situation.

So two questions...was the line crossed?? And would it have been crossed had it been the male coworker under the desk?? I feel like it was, b/c her reaction may have been veiled...hard to know for sure. But the second question I'm very unsure about.