She's not familiar with the concept of slave soldiers, but maybe that's because their nation does not employ them in that manner. Or more likely this is about the fact that she's a fairly attractive young woman and the battlefield full of armed (and likely horny) men is a very bad place to be for her, who has no known combat training or fighting skills whatsoever.
Maybe he should've considered making a descendant before he leaves for war and never returns. Then again, he's not a noble so there's really no prestigious family name that he has to pass on or anything. Even a knight counts as a low-ranking noble, but as a commoner that means he's just an ordinary foot soldier. Let's not forget she's also a slave who technically can not say no to him.