Ironically, there’s nothing in the Bible that explicitly calls out premarital sex as forbidden for Christians. It mentions “sexual immorality” and adultery, but doesn’t specify what it considers “immoral”. Even in the OT, if a man seduced an unmarried woman he either had to marry her or pay her bride-price. Also, Mary was engaged to someone else when God knocked her up.

You don't need a law. The US has codified protected classes through quite a bit of different areas like housing, employment, and education; they are race, religion, sex (and factors such as pregnancy, sexual orientation, and gender identity (but who knows with this one), national origin, age (if over 40), and disability/genetic condition. If it isn't one of those things, then selecting employees based on other traits is not discrimination in the legal sense. You can absolutely bring suit against the employer, but you are the one fighting the uphill battle arguing why unwed non-virgin should be a protected class. It's also an incredibly hard thing to pursue, because the employee must prove that is why they were fired and that alone, while the employer can just say "that and they didn't fit values/they were late/I don't like their voice". Employees don't often win.

The employee who was fired while pregnant may well have some case, because they basically admit it's due to the pregnancy, but who knows where it lands in court because the pregnancy is used as proof of other supposed rule violations.