It's bas kohen shezinsa. She has to be married.
Don't you know the case?
yes, and the Torah says she gets burnt.
This isn't a case of two punishments, like an eishes ish (chenek) who is also his mother-in-law (burnt).
This is a case that the Torah says that you get a certain punishment, even though one might have thought to give a lesser one; the Torah ramped it up to burning, much like naarah hamurasa that gets stoned, not chenek.
Its a simple chiluk, and therefore it has no bearing on the question in mind- like the case of the married shviger.