Yevamot 61 - Whom the High Priest can marry
A Kohen may not marry a woman who converted to Judaism. However, Rabbi Shimon, who usually understands and ascribes reasons for the laws of the Torah, says that the reason here is that a convert may have had relations with someone prohibited to her. Therefore, if a girl converts before the age of three, her cohabitation - even if she is violated - does not carry the disqualification power of an adult, and she can marry a Kohen.
Another lenient law from Rabbi Shimon: an idolater's grave does not spread ritual impurity under a roof, so that a person in this situation remains pure. Why? - The Torah said, "Man (Adam), when he dies under a roof...transmits impurity." And only Jews are called "Adam," because their souls all come from reincarnations of Adam's soul - which is not true for idolaters.
If a Kohen betrothed a widow (whom he is allowed to marry), and then he was appointed a High Priest, then he can go ahead and fully marry her (which would not be allowed for a regular High Priest). This is learned from the words "... only a virgin shall he take," where the words "shall he take" are extra, alluding to a special case like this where he marries a widow.
This situation actually happened with Yehoshua ben Gamla, who betrothed a widow Martha daughter of Baitos, was then appointed High Priest, but proceeded with marriage. In fact, Martha was influential in his appointment: she bought the place of High Priest for her husband with many many pounds of gold coins. But in the end Yehoshua restored Torah study and is remembered for good, as we have learned elsewhere .
Art: A Marriage Portrait by Pieter de Grebber