hmm.. Maybe it's the whole thing that men can take the beating? like in the movies a lot of men die even though the scenario would take both gender lives equally? in jurassic world there was a certain death with a woman not gonna spoil it anymore if you haven't seen it and some people thought it was tasteless because it happened to a woman.
and you know titanic? women and children first? I know that happened some time ago, but I think those kind of ideas are still a little bit in the air that men should sacrifice themselves and women should be rescued first.
maybe expendable and disposable because most of the soldiers are men? and soldiers are kinda expected to die or something
maybe they're disposable because they are good for nothing? or because it's okay to sleep with a guy or even rape him because men are monsters and that way expendable? I dunno. I'm running out of ideas.
I'm not exactly sure how the OP meant the disposable thing, just offering my thoughts.

I think "women and children" is a thing because men have the most chance of surviving in case of shit going down without lifeboats. Imagine how many kids would have died if men went first. Of course I'm not saying they're disposable but during emergencies weaker people are usually top priority.

On the Titanic part, the "women and children" was actually a result of a misheard order, I believe. They don't really do that sort of thing.

I thought it was surprising because I didn't think Disney would show someone die in an explosion. He was likeable as a character, and through the protagonist's eyes, a role model.

And not just his death, but seeing Hiro's reaction to his death which was so serious and sad (I can't say whether it was realistic or not, idk), it was surprising how dark Disney got. Not wearing other people's faces dark, but as for Disney standards, super serious.