Even if I buy the first part, we're talking about rape and not murder, so what does it have to do with whether or not the victims are "disposable"? I don't think the problem people have with rape is that it stops people from furthering society, it's that it's done without consent and can be traumatic.
Also I disagree with the first part. Men don't have wombs, but they are much less disposable as far as labor is concerned. And furthering society only matters is we care about what happens to people in society, so no one is really disposable.
The discussion wasnt about the legality or morality of rape, but about why the rape of women is treated as a more grave matter in most societies. I obviously don't think rape should be considered legal or moral, but gravity with which it is treated is informed by cultural norms.
Those norms are in turn ultimately informed by childbearing. The question was not whether someone was disposable but whether they were expendable. A hard laborer is expended for production. A soldier is expended for offense or defense. It goes without saying that disposing of a healthy man for no good reason is foolish, but expending him for some greater purpose is not. This why historically men were employed in dangerous labor or deployed as soldiers while women were not, aside from the obvious strength differential.
That difference in perceived societal value is what informs the perceived disposability of men, why they are viewed more often as tools or weapons rather than something to be nurtured and protected. This is just the way of the world. It is in the nature of a man to take the slings and arrows of the world without relying on the sympathy of others.