With manga like this one you're damned if you do and damned if you don't in regards to defending the female lead. People in real life with promiscuity similar to hers, of course, don't always (or even most of the time) have some sort of tragic backstory that explains why they turned out the way they did, but this is a fictional story, and I'm sure that the female lead's personality will have something worth understanding. Tbh I'm personally not at all fond of moral degeneracy, but people do everything for a reason.