This is something that gets brought up every time something like this happens, so I'll say it again. A lot of what people consider good marriage practice is decided by how quickly / often people die; this is further exacerbated by the realities of female biology. Once a human woman exceeds the age of 35-ish, it becomes MUCH more difficult to become pregnant: it's more difficult still to bear a child to term without crippling / lethal complications. Therefore, in a setting where people die often like this one, it is "common sense" for younger women to seek partnerships with healthy older men.
Furthermore, remember that this young lady is from an aristocratic family and that her father is planning to send (maybe sell?) her off to a faraway province. She is literally fighting for her freedom and this is her chosen method; getting hitched to someone she knows is a decent man and can have reasonable expectations of a healthy, reasonably free lifestyle with. I believe this was shown in one of the 90's chapters, you'll have to go elsewhere to find that since the previous scanlator threw a bitch fit and yanked theirs.