I REALLY hate to poke the sleeping bear, but:
I think the BIGGEST reason for the WiR phenomenon is that the majority of ongoing characters are males. Sure, there are longtime female characters, but aside from Wonder Woman, how many of them have seen continuous publication? And how many of them started as their own character, rather than as a supporting character to someone else?
My point is this. You hurt the supporting characters directly to get at the heroes. If the majority of leading characters are straight males created at a time when it was perfectly acceptable for the majority of leading characters to be straight males, then it stands to reason that a good percentage of their loved ones/supporting cast are going to be females. So now, today, we're working with well-established leading characters like Superman, Batman, Green Lantern, Green Arrow, Aquaman, Thor, Captain America, Iron Man, Wolverine, Spider-Man, etc etc etc...
Why did Mariko die?
Why did Gwen Stacy die?
Why did Supergirl die?
Because they were supporting characters/loved ones to the male leads, and there needed to be a dramatic punch to the gut of the male lead, and so the supporting cast member/loved one was killed off. It's simple statistics and logic. Does it suck? Sure, I guess. But it's not misogyny. It's story telling, working with what exists.
I'm not even listing Barbara, because her being shot as a loved one of a leading character was the same as Jason Todd being killed as the loved one of a leading character. Neither saved the day. Both were unceremoniously put down by the Joker as a means for a writer to effect change within the life of the leading character - Batman. Whether you like TKJ or not is totally a matter of personal opinion. I've never read it, personally. Didn't care to. But the point is, this is why supporting characters exist - to create drama within the lives of the leads. The lion's share of supporting characters are women, because the lion's share of established leads are men.
So yes, you're absolutely right when you say something like "This sort of thing happens to female characters more often than male characters" but you're not considering why.
It's statistics. Not misogyny.
Remember, before the Green Goblin ever snapped Gwen Stacy's neck, Uncle Ben ate a bullet just so Peter could learn a lesson about responsibility. He wasn't being a hero. He was just serving his purpose as stage dressing. To imply that female supporting characters should be above that, simply because there's been more of them over the years, is asking for special treatment in the name of equality, which has always confounded me to no end.
Next month, we can introduce Tony Stark's long lost brother, whom he can bond with, so that the next time we need to hurt Iron Man, we can attack a male instead of a female.