A woman who is a ‘zona’ or a ‘chalala’ he [a Kohen] shall not take,
The intrinsic spiritual nature of a Kohen requires protection. The Creator deemed it necessary that a Kohen’s mate be of pure lineage and unblemished background.
A Kohen may not marry any of the following women (in addition to those women forbidden to all Jewish men):
A Convert (giores) – a women born of non-Jewish parents, though she was converted Halachically to Judaism
A Zona – a Jewish women who had intimate relations with a man whom she is forbidden to marry, e.g. a non-Jew.
A Divorcee (gerusha) –a women who was married and divorced in accordance with Jewish law. This includes a women who was previously the Kohen’s own wife.
A Chalala – a Kohen’s daughter born to a women who was forbidden to him (e.g. a divorcee) or a Kohen’s daughter who had relations with a man forbidden to her (e.g. a non-Jew).
A Hostage (shevuya) –a victim of non-Jewish captors, kidnappers or the like.
This is a rabbinical prohibition while the other restrictions are from the Torah.
The Halacha requires a Kohen to separate himself immediately from any forbidden mate. A Kohen who violates any of these restrictions loses his privileges to be called first to the Torah and the right to give the Kohanim’s Blessing until he divorces the forbidden mate and vows to refrain from marrying anyone forbidden to him.
A son born to a Kohen and a woman forbidden to him is a chalal and his status as a Kohen has been profaned. Likewise, the children of a chalal are no longer treated as Kohanim.
“All Jewish families have a presumption of marital fitness (chezkas kashrus) “ (Rambam; Shulchan Aruch). However, due to the length of the exile and assimilation, today it is important to check lineage as thoroughly as possible.
The son of a known Kohen and a fit mother does not have the option of resigning from the Kehuna, and remains responsible to protect his status and that of his children.
The daughter of a Kohen (Bas Cohen) also has an elevated status. She should preferably marry a Kohen or a Torah Scholar. Unlike the general case, if the daughter of a Kohen gave birth to a firstborn son who fathered by a man forbidden to her, that son requires a redemption, Pidyon haBen for she has profaned her status.
A Kohen must be careful in selecting a proper mate, so as to pass his line and tradition to his children, and not to cause a break in the genealogical chain from Aharon haKohen.
As these matters are critical and sometimes difficult, it is necessary to consult a qualified Rabbi for advice.