The source of marriage restrictions placed on the kohen is found in vayikra;
וַיֹּאמֶר ה’ אֶל מֹשֶׁה אֱמֹר אֶל הַכֹּהֲנִים בְּנֵי אַהֲרֹן וְאָמַרְתָּ אֲלֵהֶם .. אִשָּׁה זֹנָה וַחֲלָלָה לֹא יִקָּחוּ וְאִשָּׁה גְּרוּשָׁה מֵאִישָׁהּ לֹא יִקָּחוּ כִּי קָדֹשׁ הוּא לֵאלֹהָיו.
And Hashem spoke to Moshe; Speak to the kohanim the sons of Ahron, and say to them.. a woman (who is a) harlot, a challalah they shouldn’t take and a woman who is divorced from her husband they shouldn’t take since he is holy to his G-d.
The torah thus forbids three types of women to the kohen, the zonah (Harlot), the challalah and the divrocee.
Chazal add the convert and chalutzah to the three women the torah text lists on the premise that they are derivatives of the zonah and divorcee, respectively. This brings the total count to five.
The zonah mentioned in the torah has additional far-reaching implications understood by chazal and the rishonim, most notably that even non-consentual relations with another partner would render the eshet kohen forbidden to her kohen husband (although she is entitled to her ketubah should this occur).
It is this torah law that forbids a woman who had relations with the following from becoming the legal wife of a kohen – even if the relations where non-consensual;
- A challal
- A mamzer
- An idol-worshipper
- A slave
Although a kohen is permitted to marry a widow, he is forbidden from marrying the widow of a challal. Generally speaking, it is important for the kohen marrying a widow to ascertain that the said woman is not a bat-challal (or any other disqualifying factor prior to her becoming a widow), as she is thus forbidden to him.
Also, a kohen is forbidden to marry a woman who’s father is non-jewish or a convert – even if the mother is jewish.
Rabbi Elazar stated that any non-marital intercourse (even with a permitted-for-marriage jewish male) would defile a woman from ever marrying a kohen – yet the halachic consensus disagrees with his view.
All places, all times
As with all mitzvot that are an “obligation of the body” (מצוות שהן חובת הגוף) these marriages are forbidden to the kohen in all times and places.
A kohen in a prohibited relationship
It occurs that one becomes more aware of his kehuna status well in to adult life and now has to grapple with his situation; divorce his forbidden wife or just live with being demoted (temporarily) until ending his forbidden marriage.