Mm... that's a complicated question. [Fugo brings one hand up to his face, idly tapping the his mouth when he thinks.] The problem is that, culturally, many humans are monogamous-- the ideal is to have one partner who you devote your entire life to.

Historically, women were under incredible pressure to find and devote themselves to a male partner; a "husband". It was considered a terrible disgrace for them to be sexually active before marriage. This shitty attitude has softened and changed somewhat in the modern era but, from my point in time, women who are sexually active outside of marriage are often looked down on while men of similar habits are praised.

Fantine went against what was considered, at the time, one of the most basic standards of society by having the child of a lover she wasn't married to. She lost her "worth" as a woman-- and with that stigma attached to her name, no man will marry her and no "respectable" place of work will hire her. It's a fucked up situation that was sadly very common when Les Misérables was written.