There are two separate facets to this question, and I will deal with each on their own, but as a preface I think that it is important to note that, like will all issues, there is always multiple different ways of seeing it. We will call these "discursive frames" since they are ways of talking about something, i.e. a discourse about that thing, as to bias that discourse into certain rhetorical directions. The manosphere has a particular word for this that I cannot remember offhand that basically says that feminists focus too much on a woman-centered view that they might miss the ways in which society "oppress" men. In a certain sense they are right, since focusing solely on a singular, and often univocal, point of view does in fact lead to a certain blindness towards the way in which society has negative affects on groups outside that framing, but, and this is an important but, if we take gender discourse as a whole we can discover how it is that normative judgements, such as how someone is considered "less than a man" for performing traditionally normative feminine functions, work to shape a prohibitive field in which only those that perform traditionally normative masculine functions can ascend to positions of power¹ within that society. We can see that any useful definition of sexism would have to include a conception of oppression in which gendered beings are oppressed only in-so-far as they are deprived of the same ability to "succeed" within a society whilst also keeping within the normative expectations of their gender--a definition which does not take into account any harmful affects of this normative construction, harms that are common to all norms. "Success" here being a culturally relative term dependent on the socioeconomic conditions underlying the common perception of an ideal life.

To the first facet!: actual physical masculine gendered bodies are considered to be "disposable" since they are the ones that are often sent to war or are otherwise placed in dangerous situations. Once again, they are right in a sense, since male bodies are often seen as those most capable of being in these dangerous situations. This norm, though, is still sexist in that it construes the female body as being more fragile, both physically and emotionally, to be able to handle the type of situations that mist commonly lead to physical and mental harm: war being a prime example of both kinds of harm. We can see this play out in the resistance, in America at least, to female bodied soldiers since they are seen as being both physically unable to keep up with their male bodied counterparts and as emotionally unable to handle the strain. This norm works to place women within the domain of the home, where their particular placement within this normative construct allows them to be better suited to the role--since, of course, this role produces and regulates the feminine gender as such. This also construes masculine genders as being less able to perform domestic roles, which leads to the idea that men can't be as good domestic workers as women, which leads to more women getting custody of their children. All of this works to place women squarely within the position of the home, which disadvantages them in the power struggle since the home is not a place of economic independence.

To the second!: men are seen as disposable in fiction because the vast majority of people that die in fiction, bad guys included, are male. Once again, there is a slight validity in this argument since male bodies are often the most often disposed of and treated as mere cannon fodder for the hero. But, we must remember that sexist norms see the masculine subject as being active whist the feminine subject is deemed passive. This means that women only play a part (an active position) in-so-far as they are related to an active (read: masculine) subject. Women don't do as much because doing isn't their normative role. This means that actively opposing the good guy is also outside our role except when this relation is subsumed under a relation to the good guy as in femme fatales and the such, since this role takes a passive interest to the masculine active role. So the disposibility of men in fiction stems from sexist norms with the negative affect towards men being that they tend to die a lot and the negative affect towards women being that are not represented except through their relation to a man.

1. "Power" here is being used in the non-Foucauldian sense to mean societal positions in which people can exercise control over another due to their inherent authority within that position, such as an executive being able to make decisions about a company that affect all the workers within that company. A Foucauldian/Butlerian conception of power would take power as a discursive force which creates and then regulates subjects according to the norms inherent to their creation, such as how gendered subjects are created performatively through the performance of gendered norms which produce and regulate them.