The Torah in Parashat Emor (Vayikra 21:14) lists several types of women whom a Kohen Gadol is not permitted to marry, including an Almana (widow). Due to the various prohibitions that apply regarding whom a Kohen Gadol may marry, it is possible for a Kohen Gadol to transgress four prohibitions at once, by marrying a woman and then cohabiting with her. This can happen if a widow remarried, received a divorce, and then married a Halal (somebody who is disqualified as a Kohen, such as the product of a marriage between a Kohen and a divorcee). She is now forbidden to marry a Kohen Gadol for three reasons: she is a widow, a divorcee, and a Halala (a woman who had relations with a Halal). If she then marries and is unfaithful to her husband, she is considered a “Zona” and thus forbidden for a Kohen Gadol for yet a fourth reason. If she later marries and cohabits with a Kohen Gadol, he is guilty of four transgressions, and thus liable to four sets of Malkut. The reason underlying this prohibition, as the Sefer Ha’hinuch explains, is because a widow is likely to have memories and thoughts of her first husband. The Midrash comments, “A young man who marries a widow – three minds are in the bed.” As the Kohen Gadol is to maintain an especially high standard of piety and sanctity, everything regarding his relationship his wife must be pure and pristine, and so the Torah forbade a Kohen Gadol from marrying a widow. If a woman was only betrothed (through Halachic Kiddushin) before the groom died, and the marriage was not completed, she is nevertheless considered an “Almana” and is forbidden to marry a Kohen Gadol. If a Kohen Gadol’s brother died without children, the Kohen Gadol cannot perform the Misva of Yibum by marrying his widowed sister-in-law, as he is forbidden from marrying a widow. Instead, he performs the “Halisa” ceremony whereby the widow becomes permitted to marry anyone she wishes. If a woman was betrothed, but there was some question concerning the validity of the betrothal, and the groom then died, she has the status of “Safek Almana” – meaning, it is uncertain whether or not she is a widow. In such a case, we apply the case of “Safek De’Orayta Le’humra” – that we must act stringently in situations of uncertainty regarding a Torah law (as opposed to laws enacted by the Rabbis). Hence, she may not marry a Kohen. If a Kohen marries and cohabits with a widow, he violates not only this prohibition, but also the command, “Ve’lo Yehalel Zar’o Be’amav” (Vayikra 21:15), which forbids a Kohen from having a relationship which disqualifies the woman or the offspring from the priesthood. By marrying a widow, the Kohen Gadol disqualifies her from later marrying even a regular Kohen, and so he transgresses the prohibition of “Ve’lo Yehalel.” If a Kohen has a relationship with a widow without marrying her, then he does not transgress the prohibition against marrying a widow, since they had not married, but he does transgress the prohibition of “Ve’lo Yehalel.”