"Lighten up"

No? I didn't think so. Me neither. So what are you really arguing about here?

She pointed out a lot of sexist bullshit she had to deal with. The low-cut-dress thing was just one of them. Are you now a little more alert to the fact that what you might have thought was an innocuous joke actually made a teammate feel like they have a target on their back? I am! I thought to myself, "wow, I can imagine people I've worked with in the past who would say something like that; holy shit, I might have even laughed!". Maybe you too? Then she did you a favor by speaking up.

It'd be good if people would stop expecting the Magna Carta from posts like this.

You can be friendly and still express disapproval.

Context does matter, women do have more latitude in making that kind of joke, but it still isn't something I like having reinforced (lots of people suck at context).

Why would they do it on-purpose/non-ironically/non-sarcastically?

I've also known women to do it thinking that it will help them fit in. To be one of the guys. It really sucks to be excluded.

Women can also behave badly just like men do. If you work in a sexist environment, you can pick up those attitudes and become tone-deaf. Maybe they don't realize how much it can hurt other women to reinforce that sort of idea.

There is a big difference underneath the surface, though on the surface, many such incidents look like just "joking around." The difference has to do with social distance. If you feel an off-color remark is appropriate for someone you barely know, then I hate to tell you, but you're on one end of an asymmetric power relationship, and the other party has the short end of it.

It used to be, back in the 50's, men in the US felt entitled to join conversations with and sit at the same table with a pair of young women they didn't know and had just met. That sort of thing is a clear indication of the uneven power relationship. To really understand, you need to be on the "short" end, and more, you need to understand what it's like growing up with that constantly in the environment.

Yes, often one doesn't have bad intentions. But I'm explaining it to you now. Just be honest with yourself about social distance and "kidding." If you're really self aware, you may start to realize there's been a difference in social distance between off-color remarks to women vs. others. (African Americans, perhaps?)

Of course, every workplace is different and I'm sure there are plenty of places where women don't have to deal with the bullshit described in this article. However, they are far from the norm. I can see why the OP would want to leave the industry rather than be the person constantly reporting people to HR and lecturing them about proper behavior. It may be the right thing to do, but it's also extremely uncomfortable and just as likely to create an even more hostile working environment than before.